When we added caching for protected branch names in
https://gitlab.com/gitlab-org/gitlab/-/merge_requests/64738, we did not
account for UTF-8 characters being used in the branch name.

The cache key would be encoded in ASCII-8BIT due to the branch name
coming from Gitaly, but when the performance bar were enabled the Redis
activity would be dumped to JSON. Since this string wasn't UTF-8, Ruby
would throw up a `Encoding::UndefinedConversionError`.

This commit fixes the problem by hashing the ref name with SHA-1 to
avoid needing to deal with any encoding issues.

Relates to https://gitlab.com/gitlab-org/gitlab/-/issues/343698

Changelog: fixed