Use h2 headings in PR template

Change the PR template section headings from h4 (####) to h2 (##) so they render as proper second-level sections.

Author: labkey-martyp

.github/pull_request_template.md

@@ -1,14 +1,14 @@
-#### Rationale
+## Rationale
 <!-- Rationale describing why this pull request is needed, what behavior it's adding/changing/removing, etc. (replace this comment) -->
 
-#### Related Pull Requests
+## Related Pull Requests
 - <!-- list of links to related pull requests (replace this comment) -->
 
-#### Changes
+## Changes
 - <!-- list of descriptions of changes that are worth noting (replace this comment) -->
 
 <!-- list of standard tasks (remove this comment to enable)
-#### Tasks 📍
+## Tasks 📍
 - [ ] Claude Code Review
 - [ ] Manual Testing
 - [ ] Test Automation