Use Labkit for checking and enabling FIPS

This pulls in the changes in
https://gitlab.com/gitlab-org/labkit-ruby/-/merge_requests/90, since
gitaly-ruby will also need to do something similar to enable FIPS.

This also fixes an issue with custom bit lengths for `Digest::SHA2`
not working.

Relates to https://gitlab.com/gitlab-org/gitaly/-/issues/4269

Changelog: changed