Remove rainbow requires

What does this MR do and why?

Removes rainbow/ext/string requires in the following files:

- Rakefile
- lib/gitlab/task_helpers.rb
- spec/spec_helper.rb
- spec/db/production/settings_spec.rb
- scripts/regenerate-schema
- bin/pngquant
- gems/gitlab-backup-cli/lib/gitlab/backup/cli.rb

Related to #458102 (closed)

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