Add pull request template

Mostly as I have a tendency to forget to change the target branch to the oldest supported version for a PR when I haven't worked on this repo for a little while ... oops...

Let's hope this reminder will help prevent such snafus in the future.

Ref: https://docs.github.com/en/communities/using-templates-to-encourage-useful-issues-and-pull-requests/creating-a-pull-request-template-for-your-repository

Author: jrfnl

.github/pull_request_template.md

@@ -0,0 +1,3 @@
+<!--
+REMINDER: Please target the **oldest** branch of PHPCSDevTools that is affected by this issue.
+-->