Upgrade Walter to .NET 10

[srkirkland]

Context

Walter currently targets .NET 8 across the web app, server.core, tests, and the notifications worker. During the Razor/MJML notification rendering work, we found that the current template repo pattern uses Razor.Templating.Core 3.1.0, but that package targets .NET 10 only. Walter is using Razor.Templating.Core 2.1.0 for now to stay compatible with net8.0.

Upgrading the solution to .NET 10 would let Walter align with the current web-app-template dependency stack and reduce version drift for Razor/MJML rendering.

Scope

• Update project target frameworks from net8.0 to net10.0 where appropriate.
• Update SDK/runtime expectations in local setup, CI, Azure Functions worker deployment, and docs.
• Revisit package versions after the framework upgrade, including Razor.Templating.Core.
• Validate the ASP.NET server, server.core, tests, and web/workers/notifications Azure Functions isolated worker.
• Check Azure hosting/runtime support before deployment changes.

Acceptance criteria

• dotnet test web/app.sln passes on .NET 10.
• dotnet build web/app.sln builds the web app and notifications worker on .NET 10.
• CI/build pipeline installs or uses the correct .NET 10 SDK.
• Azure deployment/runtime settings are updated or explicitly confirmed compatible.
• Notification rendering still passes focused Razor/MJML tests after dependency upgrades.
• Documentation notes the required .NET SDK/runtime version.