Streamline MQ to run only relevant GHA jobs

[timflannagan]

We recently enabled the MQ for this repository and fixed any misc. issues (e.g. #10426) that have popped up the past week. Now that the MQ is working as expected, we can fine-tune any required merge group jobs to make it quicker for PRs in the queue to merge. See https://github.com/k8sgateway/k8sgateway/actions/runs/12756850152/job/35557047075 for a concrete example where we can add some conditional logic that only runs the k8s conformance suite job instead of the rest of the workflows jobs. This may help alleviate some issues in CI where jobs are waiting for runners to be available.

[timflannagan]

Looks like the conditional approach should work according to https://stackoverflow.com/questions/76655935/when-does-a-github-workflow-trigger-for-merge-group-and-is-it-restricted-by-bran and https://github.com/orgs/community/discussions/51120#discussioncomment-6312578.