Follow-up from "Implement restore subcommand of gitlab-backup-cli": remove external require for Rainbow

The following discussion from !151996 (merged) should be addressed:

• @aakriti.gupta started a discussion:

TODO: currently we're using a lot of legacy code from lib/backup here which
# requires "rainbow/ext/string" to define the String#color method. We
# want to use the Rainbow refinement in the gem code going forward, but
# while we have this dependency, we need this external require