Update merge-queue gated feature for GHES 3.12

[willsmythe]

Why:

The "Who can use this feature" section for Merge Queue related docs (when viewing GHES 3.12) is confusing since it currently only mentions Enterprise Cloud. For example:

https://docs.github.com/en/[email protected]/repositories/configuring-branches-and-merges-in-your-repository/configuring-pull-request-merges/managing-a-merge-queue

This change updates the gated feature details to be correct for GHES.

Note: the MQ feature is currently gated to GHES 3.12 and later.

What's being changed (if available, include any code snippets, screenshots, or gifs):

Fixes the "Who can use this feature" section on Merge Queue related pages when viewing GHES 3.12 and later docs.

Check off the following:

• I have reviewed my changes in staging, available via the View deployment link in this PR's timeline (this link will be available after opening the PR).

  • For content changes, you will also see an automatically generated comment with links directly to pages you've modified. The comment won't appear if your PR only edits files in the data directory.

• For content changes, I have completed the self-review checklist.

[willsmythe]

I verified the correct message still appears for Cloud:

https://docs-31946-d0a842.preview.ghdocs.com/en/pull-requests/collaborating-with-pull-requests/incorporating-changes-from-a-pull-request/merging-a-pull-request-with-a-merge-queue

And the correct message now appears for GHES 3.12:

https://docs-31946-d0a842.preview.ghdocs.com/en/[email protected]/pull-requests/collaborating-with-pull-requests/incorporating-changes-from-a-pull-request/merging-a-pull-request-with-a-merge-queue

[isaacmbrown]

Looks good, thanks @willsmythe 🙇

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