[Clean-up] Centralize LangVersion to 10

We specify the `LangVersion` globally here:

https://github.com/CommunityToolkit/Labs-Windows/blob/60927b0c5975e1ccdbf52a899e044109aca1df1f/Windows.Toolkit.Common.props#L20

(Though maybe this should just be in the `Directory.Build.props` instead?)

We should update this to be explicitly 10.0.

We should also remove it from being duplicated in the two SG projects so we can manage it centrally:

https://github.com/CommunityToolkit/Labs-Windows/blob/63a00ffa3a4b69da3c68f5d090ad4e0de5eae350/common/CommunityToolkit.Labs.Core.SourceGenerators/CommunityToolkit.Labs.Core.SourceGenerators.csproj#L7

https://github.com/CommunityToolkit/Labs-Windows/blob/18fe7e7d929d6f4545eb7a095b0478ddffd0e3e9/common/CommunityToolkit.Labs.Core.SourceGenerators.XamlNamedPropertyRelay/CommunityToolkit.Labs.Core.SourceGenerators.XamlNamedPropertyRelay.csproj#L7

Read Windows.Toolkit.Common.props at the linked LangVersion entry, then inspect the LangVersion entries in the two CommunityToolkit.Labs.Core.SourceGenerators .csproj files. Confirm the intended central location, update the global value to explicit 10.0, remove the duplicates, and verify the affected projects still build.

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