Remove tarball support from SB release pipeline

[mthalman]

The release pipeline currently supports tarball-based versions (e.g. 6.0) and VMR-based versions (e.g. 8.0). Once 6.0 is EOL, we should update the pipeline to only support the VMR configuration.

[dotnet-issue-labeler]

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.

[dotnet-issue-labeler]

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.

[MichaelSimons]

This cleanup should include the archiving/removal of the dnceng security-partners-dotnet repo that is used as part of the release process.