Allow approving Pull Requests after they are merged

Currently, a Pull Request cannot be approved if it is merged. The Approve button goes away post merge.

There are some use cases where the Pull Request needs an approval button after the merge from some of our customers. For example -

"For us, sometimes we merge some PRs and feedback reviews are happening after, for instance, when we're in a hurry for a hotfix, there's only one developer available on support call duty and we do need to deploy that hotfix quickly into production, however reviewers are doing the reviews after (the next day) to be sure the new code is correct and comply with the rest of our platform.

In case it's not correct we don't approve it (big green button is not there) and we make comments to address code changes in a new PR. If everything is ok we approve the PR and the big green button is visible for everyone, so the rest of developers on that team and the rest of the reviewers can verify that it was approved for the others stakeholders even after the merge."

Latest update from Atlassian 2023-02-28
This is now resolved. We have reinstated the ability for pull request reviewers to give approval after a PR is closed. Please see our comment below if you are interested in the history on this issue.