Fix RS1038 Warning: Compiler extensions should be implemented in assemblies with compiler-provided references

[TheCodeTraveler]

Description of Change

This PR adds RS1038 to <WarningsAsErrors> and replaces Microsoft.CodeAnalysis.CSharp.Workspaces -> Microsoft.CodeAnalysis.CSharp

Link to RS1038 Docs:
https://github.com/dotnet/roslyn-analyzers/blob/main/docs/rules/RS1038.md

Additional information

Link to Compiler Warnings seen in CommunityToolkit.Maui: https://github.com/CommunityToolkit/Maui/actions/runs/15263232470/job/42928522628#step:14:61

Warning: D:\a\Maui\Maui\src\CommunityToolkit.Maui.Analyzers\UseCommunityToolkitInitializationAnalyzer.cs(9,2): warning RS1038: This compiler extension should not be implemented in an assembly containing a reference to Microsoft.CodeAnalysis.Workspaces. The Microsoft.CodeAnalysis.Workspaces assembly is not provided during command line compilation scenarios, so references to it could cause the compiler extension to behave unpredictably. (https://github.com/dotnet/roslyn-analyzers/blob/main/docs/rules/RS1038.md) [D:\a\Maui\Maui\src\CommunityToolkit.Maui.Analyzers\CommunityToolkit.Maui.Analyzers.csproj]

[Copilot]

Pull Request Overview

This PR ensures compliance with the RS1038 analyzer by removing the workspaces package reference and treating RS1038 as an error.

• Swap Microsoft.CodeAnalysis.CSharp.Workspaces for Microsoft.CodeAnalysis.CSharp at 4.14.0
• Add RS1038 to the <WarningsAsErrors> list and adjust the surrounding XML comment

Reviewed Changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated no comments.

File	Description
src/CommunityToolkit.Maui.Markup.SourceGenerators/CommunityToolkit.Maui.Markup.SourceGenerators.csproj	Replaced Microsoft.CodeAnalysis.CSharp.Workspaces with Microsoft.CodeAnalysis.CSharp
Directory.Build.props	Added RS1038 to WarningsAsErrors and moved the XML comment closing tag

Comments suppressed due to low confidence (1)

Directory.Build.props:70

• The XML comment closing marker (-->) is on the same line as the RS1038 entry; please verify that the opening <!-- is correctly paired so that the warning entry is not accidentally commented out or causing an XML parse error.

RS1038: Compiler extensions should be implemented in assemblies with compiler-provided references-->