Tasks needed for .NET 9 GA

[lbussell]

• Move latest tags in manifest.json to the new version
  - Relevant issue: Tests to validate floating tags #2316
• Update documentation as necessary to reference .NET 9.0 #5982
• Add .NET 9 to .NET Framework SDK images microsoft/dotnet-framework-docker#1180
  • Add .NET 9 to SDK images microsoft/dotnet-framework-docker#1181
• Post a new announcement on the GitHub Discussions page page and pin the announcement
  • Include new .NET Docker features and breaking changes and defer to other official .NET documentation/announcements for other features
  • Example announcement: .NET 8.0 Container Images Now Available #4995

I couldn't figure out the best area label to add to this issue. If you have write-permissions please help me learn by adding exactly one area label.

[lbussell]

.NET 9 announcement: #6050