Change the first heading in the pull request template to be useful

# 💡 Summary #

Per https://github.com/cisagov/.github/pull/28#pullrequestreview-824239406:

> One thing I would like to see addressed is the octothorpe-wrapped comment at the very top of the PR template. All that line does is add a pointless horizontal line at the top of the PR, which I find maddening. We can't just remove the octothorpes, since then the linters complain that the first heading in the file isn't a first-level heading. We need to do one of the following:
>
>- Add _something_ to that first-level heading, but I can't think of anything meaningful. The PR title is already handled by the PR web page, so it doesn't make sense to put it there.
>- Make the other headings all first-level headings, but that looks terrible.

## Motivation and context ##

As the above remarks, the empty first heading in the pull request template is functionally useless at present.

Open the pull request template and inspect its empty first-level heading, along with the later headings and the referenced review discussion in pull request 28. Choose a meaningful first-level heading that avoids the pointless horizontal line while keeping the template compliant with the linters; done means the heading is useful and the template still renders acceptably.

Written by the indexing model from the issue text.