Release notes check + merge queue

[mfocko]

As reported by @lbarcziova, right now it is not possible to use merge queue with the release notes check

• can be seen here
• relevant docs here

We need to react on merge_groups (checks_requested) events too. Event should contain merge_group.head_commit.message which should contain the release notes. If it is the merge commit :)

[mfocko]

Since this has a low priority and doesn't block us, moving to backlog for “some” (hopefully, potentially, perhapsly) Friday.