Remove CBL Mariner floating tags for .NET 9

[lbussell]

Describe the Problem

.NET 9 only supports one version of CBL Mariner. In order to bring it in line with our other distro offerings like Ubuntu, we should remove the cbl-mariner floating tag for .NET 9 images.

Additional Context

Although .NET 8 also only supports one version of CBL-mariner, we need to leave the tag to avoid breaking anyone that's using it.

[lbussell]

[Triage] For .NET 8.0 CBL Mariner floating tags, it doesn't make sense for them to exist given that .NET 8 only supports one version of Mariner. We should avoid adding the floating tags for any new images and make the current floating tags undocumented to avoid confusion in the future. This should be logged as a separate issue.

[lbussell]

We should also consider undocumenting the tags for 6.0 and 7.0 as simplifying the tag listing can benefit all the versions of .NET.

[richlander]

.NET 9 only supports one version of CBL Mariner.

This isn't really the reason. In general, we don't want to make it easy (with tags) to jump from one OS version to the other. It will only result in "compatibility tears". The exception is Alpine since they are on a different plan and we're just playing along with that.

[lbussell]

Filed https://github.com/dotnet/dotnet-docker/issues/5292 as follow up according to https://github.com/dotnet/dotnet-docker/issues/5268#issuecomment-1998083321.

[lbussell]

This is completed by the above linked PRs.