Merge Check: Prevent PRs from being merged if there are open conversations

[bkhouri]

In order to ensure all comments were at least read by the author, the project should prevent PRs from being merged if there are open conversations.

It appears GitHub has support for this (See step 8 in https://docs.github.com/en/repositories/configuring-branches-and-merges-in-your-repository/managing-protected-branches/managing-a-branch-protection-rule#creating-a-branch-protection-rule)

This should apply to all PR that have a destination branch of a release/* branch, including main