serviceconfig: move release level to `api.go` since this is language neutral

I think it’s fine to put these here for now, but we should move them to api.go at some point since these values should be language-neutral.

_Originally posted by @julieqiu in https://github.com/googleapis/librarian/pull/3952#pullrequestreview-3770263446_
            