Delete request to manifest tag should return 202, unless tag deletion is disallowed (400/405)

Context

Related to &10345 (closed).

Problem

After adding OCI conformance tests to the CI pipeline, we have identified this failure:

------------------------------
• Failure [0.009 seconds]
OCI Distribution Conformance Tests
/builds/gitlab-org/container-registry/distribution-spec/conformance/00_conformance_suite_test.go:12
  Content Management
  /builds/gitlab-org/container-registry/distribution-spec/conformance/04_management_test.go:13
    Manifest delete
    /builds/gitlab-org/container-registry/distribution-spec/conformance/04_management_test.go:86
      DELETE request to manifest tag should return 202, unless tag deletion is disallowed (400/405) [It]
      /builds/gitlab-org/container-registry/distribution-spec/conformance/04_management_test.go:87
      Expected
          <string>: DIGEST_INVALID
      to equal
          <string>: UNSUPPORTED
      /builds/gitlab-org/container-registry/distribution-spec/conformance/04_management_test.go:101
------------------------------

This is due to the fact that the OCI Distribution Spec v1 has introduced a tag delete functionality under the /v2/<name>/manifests/<tag> endpoint, which we don't support. We added support for tag deletes way before, but we did so using the /v2/<name>/tags/reference/<reference> endpoint.

Considering this, we should:

1. Add support for tag deletion under the DELETE /v2/<name>/manifests/<tag> endpoint, replicating what we already do in DELETE /v2/<name>/tags/reference/<reference>;
2. Deprecate support for the DELETE /v2/<name>/tags/reference/<reference> endpoint;
3. Raise an issue so that GitLab Rails starts using DELETE /v2/<name>/manifests/<tag> instead of DELETE /v2/<name>/tags/reference/<reference>;
4. Raise an issue to remove the DELETE /v2/<name>/tags/reference/<reference> endpoint in %17.0, marking it as blocked by the previous issue.

https://gitlab.com/gitlab-org/container-registry/-/jobs/4803030807