Clarify "external registry" is actually "third-party"

What does this MR do and why?

This documentation describes

Using external container registries in GitLab is deprecated in GitLab 15.8 and the end of support is scheduled for GitLab 16.0. If you need to use external container registries instead of the GitLab container registry, tell us about your use cases in feedback issue 958.

This statement has two points to clarify.

• The "external" in this context actually means "third-party". On the other hand, GitLab still supports to externalize GitLab's container registry. It is misleading statement so need to clarify that means third-party. (internal issue: https://gitlab.com/gitlab-com/ops-sub-department/section-ops-request-for-help/-/issues/283)
• GitLab 16.0 is already released. So the "end of support is scheduled" should be changed to past tense.

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