[Feature flag] Cleanup package_registry_access_level

<!-- Title suggestion: [Feature flag] Cleanup package_registry_access_level --> ## Summary This issue is to cleanup the `package_registry_access_level` feature flag, after the feature flag has been enabled by default for an appropriate amount of time in production. <!-- Short description of what the feature is about and link to relevant other issues. --> ## Owners - Team: Package Registry - Most appropriate slack channel to reach out to: `#g_package-registry` - Best individual to reach out to: @10io - PM: @trizzi - EM: @ckroutil ## Stakeholders <!-- Are there any other stages or teams involved that need to be kept in the loop? - Name of a PM - The Support Team - The Delivery Team --> ## Expectations ### What might happen if this goes wrong? Issues with the permissions of the Package Registry and/or the `allow anyone to pull` setting. ### Cleaning up the feature flag <!-- The checklist here is to help stakeholders keep track of the feature flag status --> - [x] Create a merge request to remove `package_registry_access_level` feature flag. Ask for review and merge it. - [x] Remove all references to the feature flag from the codebase. - [x] Remove the YAML definitions for the feature from the repository. - [x] Create [a changelog entry](https://docs.gitlab.com/ee/development/feature_flags/#changelog). - [x] Ensure that the cleanup MR has been deployed to both production and canary. If the merge request was deployed before [the code cutoff](https://about.gitlab.com/handbook/engineering/releases/#self-managed-releases-1), the feature can be officially announced in a release blog post. - [x] `/chatops run auto_deploy status <merge-commit-of-cleanup-mr>` - [x] Close [the feature issue](ISSUE LINK) to indicate the feature will be released in the current milestone. - [x] If not already done, clean up the feature flag from all environments by running these chatops command in `#production` channel: - [x] `/chatops run feature delete package_registry_access_level --dev` - [x] `/chatops run feature delete package_registry_access_level --staging` - [x] `/chatops run feature delete package_registry_access_level` - [x] Close this rollout issue.

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