Add a PR checklist item for "I have read the contributing guide..." as a catch-all

[dbeatty10]

Describe the "under the hood" change

Our PR template is pretty long, which makes it onerous to use. These items are already covered in our contribution guidelines in their own section:

So we should remove these (4) checklist items for a (1) checklist item that says:

• I have read the contributing guide and understand what's expected of me

Then the PR reviewer can just link to that section of the contribution guide as-needed.

Who will this benefit?

This will reduce the length of the PR checklist, which will make it easier to complete (in alignment with the suggestions within the Checklist Manifesto).

This will also more closely align the PR template within dbt-utils with the PR templates of dbt-core, dbt-adapters, etc.