Enables frozen string for the following:

* lib/gitlab/hook_data/**/*.rb
* lib/gitlab/i18n/**/*.rb
* lib/gitlab/import/**/*.rb
* lib/gitlab/import_export/**/*.rb
* lib/gitlab/kubernetes/**/*.rb
* lib/gitlab/legacy_github_import/**/*.rb
* lib/gitlab/manifest_import/**/*.rb
* lib/gitlab/metrics/**/*.rb
* lib/gitlab/middleware/**/*.rb

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

This reverts merge request !20801

This is refactoring in the lead up to passing mutual TLS certs for helm applications. As such we expect all applications to need config files so we can remove the logic about which applications need and do not need this (ie `#config_map?`).