Enable some frozen string in ee/lib

Enables frozen string for the following:

• ee/lib/gitlab/**/*.rb
• ee/lib/omni_auth/**/*.rb
• ee/lib/pseudonymizer/**/*.rb
• ee/lib/system_check/**/*.rb

What are the relevant issue numbers?

Partially addresses gitlab-org/gitlab-ce#47424.

Suggested Reviewers/Approvers/Mergers

@rymai @godfat @stanhu