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.