build(dependabot): ignore minor and patch github-actions updates

[Fdawgs]

Resolves issue raised in #65, adapted from example here.

Checklist

• commit message and code follows the Developer's Certification of Origin
  and the Code of conduct

[Fdawgs]

Okay so GitHub's own actions follow this tag style for major releases, so specifying actions/setup-node@v2 will get the latest v2.*.* of that action.

fastify/github-action-merge-dependabot does not follow this and does not have a v2. If I drop that back to v2.0.0 then it will stay at that version, and not automatically use v2.5.0.

To accommodate for this, I can stop minor and patch updates for only Github's own actions in the actions/ namespace, and then increase the interval from daily to monthly so we still get fastify/github-action-merge-dependabot updates but at a less frequent pace.
Alternatively, fastify/github-action-merge-dependabot needs to be updated to follow GitHub Action's tagging process.

Thoughts @jsumners, @mcollina?

[jsumners]

I really don't enjoy automatically updating the Actions's versions to begin with. So the fewer notifications from Dependabot about it, the better.