Support GitHub merge queues

Add merge_group as a valid event. Allows the action to run against
commits that are part of a merge queue.

Example failure:

github.com/submariner-io/coastguard/actions/runs/4618846627/jobs/8166807512

Signed-off-by: Daniel Farrell <dfarrell@redhat.com>

index.js

@@ -1,7 +1,7 @@
 const core = require('@actions/core')
 const github = require('@actions/github')
 
-const validEvent = ['pull_request']
+const validEvent = ['pull_request', 'merge_group']
 
 async function main() {
   try {