Update merge request template & checklist

It seems like the default review checklist for codequality merge requests could stand to be updated. Here's my initial pass over the current list:

• Changelog entry added

👍

• Documentation created/updated for GitLab EE, if necessary

1. It seems like this is specifically regarding product/output changes that would be automatically included into GitLab when a new codequality version is released. If that's the case, we could elaborate on that scenario and remove the reference to EE, because I think it would apply equally to either release version.

• Documentation created/updated for this project, if necessary

👍

• Documentation reviewed by technical writer or follow-up review issue created

2. This might only apply for GitLab documentation updates? We could also have a technical writer reviewing these project docs but it might be overkill for now. Very open to opinions here.

• Tests added for this feature/bug

👍

• Job definition updated, if necessary
  • Auto-DevOps template (also in CE)
  • Job definition example
  • CI Templates (to be removed)
  • Vendored CI Templates (also in CE)

3. I think several of these thinks don't quite point to the right place anymore. Let's check which ones still are conceptually useful, and make sure they're the right links.

• Conforms to the code review guidelines

4. The GitLab code review documentation might be too extensive to apply here. We might want to boil down our own guidelines into README.md#Contributing or something.

• Conforms to the Go guidelines

5. What?

• Security reports checked/validated by reviewer

6. I'm not sure what reports these are, it seems like this, similar to the above Go item, were copied from another project. I think we could remove these two, but we should have a public list of security concerns and a review checklist for this project.