RFR: 2234: /reviewers N should remove ready status for merge pull requests

[Zhao Song]

In this pr https://github.com/openjdk/jfx22u/pull/26, Kevin issued "/reviewers 1" in the merge pull request but skara didn't remove the ready label.

Jfx22u was not configured with “reviewMerge”, so the reviewers jcheck were skipped. The reason we can see `Change must be properly reviewed (1 review required, with at least 1 Reviewer)` in the PR body is because of [SKARA-1824](https://bugs.openjdk.org/browse/SKARA-1824).

To resolve this issue, we should enable `reviewMerge` when a reviewers command is issued in a merge pull request.

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Commit messages:
 - update
 - SKARA-2234

Changes: https://git.openjdk.org/skara/pull/1639/files
  Webrev: https://webrevs.openjdk.org/?repo=skara&pr=1639&range=00
  Issue: https://bugs.openjdk.org/browse/SKARA-2234
  Stats: 77 lines in 2 files changed: 75 ins; 2 del; 0 mod
  Patch: https://git.openjdk.org/skara/pull/1639.diff
  Fetch: git fetch https://git.openjdk.org/skara.git pull/1639/head:pull/1639

PR: https://git.openjdk.org/skara/pull/1639