Add new LangVer for C# 14 and related paperwork

[jcouv]

We usually add new language versions early August.

This list must be checked for this repo:

• update the feature status page
• update all the tests that call this canary
• replace all references to C# "Next" (such as TestOptions.RegularNext or LanguageVersionFacts.CSharpNext) with the new version
• update all the tests for newly shipped features including ErrorCode.ERR_FeatureInPreview diagnostics and misc failing tests
• update _MaxAvailableLangVersion cap (a relevant test should break when new version is introduced)
• update the "UpgradeProject" codefixer

Other repos also need updates:

• email release management to add to the release notes. See csharp-version in release.json in previous example: Artifacts for .NET 9 RC1 Release core#9493
• make csharplang updates documented at https://github.com/dotnet/csharplang/blob/main/Design-Process.md#steps-to-move-a-triaged-feature-to-an-implemented-feature
• push the list of specs to Codex. https://devdiv.visualstudio.com/OnlineServices/_git/CodexV2Data/pullrequest/660223
  See previous example: https://devdiv.visualstudio.com/OnlineServices/_git/CodexV2Data/pullrequest/618779

Also need to add new LangVer for VB.