[msbuild] Improve how we handle the path to Xcode.

[rolfbjarne]

• Add support for an MSBuild property ('XcodeLocation') to set the Xcode location.
• Use a new facility in macios-devtools (the 'XcodeLocator' class) to find Xcode. This required a bump to the corresponding submodule.
• Deprecate the file-based locations.
• Always set DEVELOPER_DIR when executing subprocesses.
• Don't use static state to store the Xcode location. This fixes a problem where the static state would remain in memory when MSBuild's build server is being used, and then we'd do the wrong thing when building a different project using a different version of Xcode.

Misc improvements:

• We now warn if the selected Xcode is a symlink (ref: ACTool task fails to build when the currently selected Xcode is a symlink #21762).
• The settings files are deprecated, and won't be used in .NET 11 (we'll warn about this in .NET 10).
• The environment variable MD_APPLE_SDK_ROOT is deprecated, and won't be used in .NET 11 (we'll warn about this in .NET 10).

This will hopefully remove some confusion about which Xcode is being used; now we just look at the system's Xcode, unless it's overridden by the MSBuild property 'XcodeLocation'.

Fixes #3931.
Fixes #11172.
Fixes #21762.

[Copilot]

Pull request overview

This PR migrates Xcode discovery and configuration away from static, file-based mechanisms and the MD_APPLE_SDK_ROOT environment variable to a new XcodeLocator-based flow, driven primarily by the XcodeLocation MSBuild property and DEVELOPER_DIR. It updates tasks, tests, and documentation to align with the new behavior and deprecation plan.

Changes:

• Introduce XcodeLocator usage into XamarinTask/DetectSdkLocation and wire SdkDevPath through MSBuild targets, removing the static Sdks helper.
• Replace MD_APPLE_SDK_ROOT usage with DEVELOPER_DIR/XcodeLocation across build scripts and tests, and add deprecation warnings for the old environment variable and settings files.
• Update tests and docs to validate the new Xcode selection behavior and reflect the new configuration options.

Reviewed changes

Copilot reviewed 24 out of 24 changed files in this pull request and generated 3 comments.

Show a summary per file

File	Description
tests/xharness/Jenkins/TestTasks/AppleTestTask.cs	Switch test harness env var from MD_APPLE_SDK_ROOT to DEVELOPER_DIR.
tests/package-mac-tests.sh	Update packaging script to export DEVELOPER_DIR instead of MD_APPLE_SDK_ROOT.
tests/msbuild/Xamarin.MacDev.Tasks.Tests/TaskTests/IBToolTaskTests.cs	Update IBTool tests to use CurrentSdk instead of the removed static Sdks helper and shared execution helper.
tests/msbuild/Xamarin.MacDev.Tasks.Tests/TaskTests/GeneratePlistTaskTests/GeneratePlistTaskTests_tvOS.cs	Use Task.CurrentSdk for tvOS default SDK version resolution.
tests/msbuild/Xamarin.MacDev.Tasks.Tests/TaskTests/GeneratePlistTaskTests/GeneratePlistTaskTests_iOS.cs	Use Task.CurrentSdk for iOS default SDK version resolution.
tests/msbuild/Xamarin.MacDev.Tasks.Tests/TaskTests/GeneratePlistTaskTests/GeneratePlistTaskTests_Core.cs	Ensure TargetFrameworkMoniker is set based on Platform in core plist tests.
tests/msbuild/Xamarin.MacDev.Tasks.Tests/TaskTests/CompileAppManifestTaskTests.cs	Adjust CompileAppManifest tests to use CurrentSdk for installed SDK versions.
tests/msbuild/Xamarin.MacDev.Tasks.Tests/TaskTests/ACToolTaskTest.cs	Update ACTool tests to create the task earlier and avoid Sdks.
tests/msbuild/Xamarin.MacDev.Tasks.Tests/AssemblySetup.cs	Use DEVELOPER_DIR instead of MD_APPLE_SDK_ROOT in MSBuild test setup.
tests/dotnet/UnitTests/XcodeVersionTest.cs	Drive Xcode selection via the XcodeLocation MSBuild property instead of MD_APPLE_SDK_ROOT.
tests/common/Configuration.cs	Set DEVELOPER_DIR instead of MD_APPLE_SDK_ROOT in common test build environment.
tests/common/BinLog.cs	Filter out deprecation noise from settings-file-related warnings in binlog parsing.
msbuild/Xamarin.Shared/Xamarin.Shared.targets	Thread SdkDevPath into tasks and pass XcodeLocation into DetectSdkLocations.
msbuild/Xamarin.MacDev.Tasks/Tasks/XamarinTask.cs	Add XcodeLocator and CurrentSdk instance-level handling and improve logging signatures.
msbuild/Xamarin.MacDev.Tasks/Tasks/ReadAppManifest.cs	Use CurrentSdk for Mac Catalyst iOS-version conversion.
msbuild/Xamarin.MacDev.Tasks/Tasks/DetectSigningIdentity.cs	Resolve codesign_allocate via CurrentSdk.DeveloperRoot.
msbuild/Xamarin.MacDev.Tasks/Tasks/DetectSdkLocation.cs	Rework SDK detection to use XcodeLocator, emit deprecation warnings, and rely on CurrentSdk.
msbuild/Xamarin.MacDev.Tasks/Tasks/CompileEntitlements.cs	Resolve default entitlements path via CurrentSdk.
msbuild/Xamarin.MacDev.Tasks/Tasks/CompileAppManifest.cs	Use CurrentSdk and XcodeLocator when validating SDKs and populating Xcode-related plist keys.
msbuild/Xamarin.MacDev.Tasks/Sdks.cs	Remove the static Sdks helper in favor of instance-based CurrentSdk.
msbuild/Xamarin.Localization.MSBuild/MSBStrings.resx	Add new warning/error messages for Xcode detection and deprecations.
external/Xamarin.MacDev	Bump Xamarin.MacDev submodule to gain XcodeLocator support.
docs/building-apps/build-properties.md	Document the new XcodeLocation build property and Xcode resolution precedence.
Make.config	Stop exporting MD_APPLE_SDK_ROOT at the root make level in favor of DEVELOPER_DIR.

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