Enables frozen string for the following files:

* lib/generators/**/*.rb
* lib/gitaly/**/*.rb
* lib/google_api/**/*.rb
* lib/haml_lint/**/*.rb
* lib/json_web_token/**/*.rb
* lib/mattermost/**/*.rb
* lib/microsoft_teams/**/*.rb
* lib/object_storage/**/*.rb
* lib/omni_auth/**/*.rb
* lib/peek/**/*.rb
* lib/rouge/**/*.rb
* lib/rspec_flaky/**/*.rb
* lib/system_check/**/*.rb

Partially addresses #47424.

Closes #44121.

Because this code resides in lib/ it may be eager loaded. Rails in turn
doesn't define Rails::Generators by default unless you explicitly
require "rails/generators".

These are regular Rails migrations that are executed by default. A user
can opt-out of these migrations by setting an environment variable
during the deployment process.

Fixes gitlab-org/gitlab-ce#22133