Currently CSharp9 is an alias for preview.

We need to make it it's own distinct language version before releasing:

• Attach C# 9 features to CSharp9 (see MessageID.cs)
• Make the default language CSharp9
• Remove the default for .netcoreapp5.0 (No longer applicable)
• Update all tests the refer to preview and make them refer to 9
• Update the UpgradeProject fixer tests in the IDE
• Remove the pseudo CSharp9 constant used in the IDE layer (see Update IDE to point at actual LanguageVersion.CSharp9 once available. #43348 and https://github.com/chsienki/roslyn/commits/remove_pseudo_langver9)
• Verify end-to-end scenario
• Remove IDE dropown comment in source as it's no longer applicable