[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