Add netstandard2.1 target and update DiagnosticSource dependency

[stevejgordon]

Fixes #2657
Fixes #2445

This adds a netstandard2.1 target so that features such as OTel bridge are available on older runtimes, even though those are out of support. This prevents an update of our package, without a runtime update, breaking/removing features. Features may still not be 100% compatible with older unsupported runtimes, but should work in general.

As part of this, I updated the minimum dependency for System.Diagnostics.DiagnosticSource which is no generally 8.0.0. We still ship a version of Elastic.Apm and loader for the Startup hooks zip file which are built against 6.0.0. These support older unsupported runtimes although with no official testing.

Compiler pre-processor directives now prefer NET over a specific .NET version. We also add NETSTANDARD2_1 so that features continue to work on older runtimes without a specific .NET target, i.e. 6 and 7.

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• docs/reference/setup-dotnet-net-core.md

[stevejgordon]

I updated to the .NET 9 SDK and that I suspect is triggering the new formatting warnings, but I'm not entirely sure why. Will work on resolving those.

[stevejgordon]

Working on the test failures. The IIS ones suggest we might still have to ship an older DiagnosticsSource build too, although I need to investigate why exactly based on our packaging process.