Rename ruby-3.0.0-* CI images to ruby-3.0
Cosmetic: Rename ruby-3.0.0-* CI images to ruby-3.0.
Both match 3.0.+ but it is confusing to ask for 3.0.0 and get 3.0.2.
Another (maybe better) option could be to change the matcher in custom-docker-build
to actually match the patch level when asking for it?