feat(branchNameStrict)!: remove forward slashes from non-prefix part of branchName (#32278)

Update logic for branchNameStrict=true so that it also removes forward slashes from branch names, with the exception of the `branchPrefix` part. In other words, if you leave the default `branchPrefix="renovate/"` then you'll get branch names like `renovate/foo-bar-42-x` instead of today's `renovate/foo/bar-42-x`.

BREAKING CHANGE: Branch names with multiple forward slashes will change if branchNameStrict=true