Remove .NET 7 from main and nightly branches

[lbussell]

.NET 7 will reach End of Support on May 14, 2024.

From the .NET major version release lifecycle checklist:

• [ ] Remove all entries for the EOL .NET version in manifest.json, manifest.versions.json, ImageVersion.cs, TestData.cs, MCR tags metadata templates, and delete the Dockerfiles
• [ ] Search for and simplify conditions including the EOL .NET Version in the Dockerfile templates and tests
• [ ] Replace all references to the EOL .NET version in documentation with a new .NET version
• [ ] Remove the EOL .NET version from the .NET Framework SDK images: https://github.com/microsoft/dotnet-framework-docker/blob/6a3c9d048f75c1e69c0e1059564cd56a90bf9e3c/eng/dockerfile-templates/sdk/Dockerfile#L56-L59
• [ ] Post a new announcement on the GitHub Discussions page