Raise the minimum Roslyn version for MSTest.Analyzers to 4.14

Nobody has claimed this yet.

Raise `MicrosoftCodeAnalysisVersion` from 3.11.0 to 4.14.0 for MSTest.Analyzers and its code fixes during the next breaking-change release.

This intentionally raises the minimum supported analyzer host to Roslyn 4.14 (Visual Studio 2022 17.14 / .NET 9.0.3xx SDK or newer). Older hosts cannot load analyzer assemblies built against Roslyn 4.14 because Roslyn is supplied by the compiler host and excluded from the MSTest.Analyzers package.

Implementation should:

- update `MicrosoftCodeAnalysisVersion` and raise `MicrosoftCodeAnalysisVersionForTests` to at least 4.14;
- adopt the newer Roslyn APIs and nullable annotations where appropriate;
- remove compatibility code that exists only for the Roslyn 3.11 build floor;
- verify the packaged analyzers and code fixes load and run on the oldest host retained by the v5 support policy;
- document the new minimum Visual Studio / .NET SDK requirement.

Reference: dotnet/sdk#55525, which makes the equivalent minimum-host change for the out-of-band Microsoft.CodeAnalysis.NetAnalyzers package.

Start by locating MicrosoftCodeAnalysisVersion and MicrosoftCodeAnalysisVersionForTests in the MSTest.Analyzers build configuration, then inspect the analyzer and code-fix compatibility paths tied to Roslyn 3.11. Verify the packaged analyzers and code fixes on the oldest host retained by the v5 support policy, and update documentation with the Visual Studio 2022 17.14 / .NET 9.0.3xx minimum.

Written by the indexing model from the issue text.