Add `merge_group` build trigger

[yeikel]

What problem are you trying to solve?

Add the merge_group trigger to a build so that builds are triggered in the merge queue

What precondition(s) should be checked before applying this recipe?

This depends on enablement of merge queue in the branch protection rules but it is out of the scope of this project

Describe the situation before applying the recipe

on:
  pull_request:

Describe the situation after applying the recipe

on:
  pull_request:
  merge_group:

Any additional context

The generic recipe https://github.com/openrewrite/rewrite-github-actions/issues/90 could be used to accomplish this but as that recipe will be more involved, we can deliver this independently first.

https://docs.github.com/en/repositories/configuring-branches-and-merges-in-your-repository/configuring-pull-request-merges/managing-a-merge-queue

[yeikel]

@timtebeek What are your thoughts about delivering this before #90?