Enable even more frozen string in lib/**/*.rb

What does this MR do?

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

This should cover all lib/**/*.rb files except for the massive lib/gitlab directory.

What are the relevant issue numbers?

Partially addresses #47424 (closed).

Suggested Reviewers/Approvers/Mergers

@rymai @godfat @stanhu