Signed-off-by: Rémy Coutable <remy@rymai.me>

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.

* Replace `require` or `require_relative` with `require_dependency`
* Remove unneeded `autoload`

See
http://guides.rubyonrails.org/autoloading_and_reloading_constants.html
for more info.

Signed-off-by: Rémy Coutable <remy@rymai.me>