Updated GitHub PR template headings to level 4.

GitHub pull request descriptions are rendered as a comment. Comment
titles, which include the PR author, render in a h3. Hence, titles
within the comment body should be header level 4. This makes pull
request descriptions more accessible to screen readers.

.github/pull_request_template.md

@@ -1,12 +1,12 @@
-# Trac ticket number
+#### Trac ticket number
 <!-- Replace XXXXX with the corresponding Trac ticket number, or delete the line and write "N/A" if this is a trivial PR. -->
 
 ticket-XXXXX
 
-# Branch description
+#### Branch description
 Provide a concise overview of the issue or rationale behind the proposed changes.
 
-# Checklist
+#### Checklist
 - [ ] This PR targets the `main` branch. <!-- Backports will be evaluated and done by mergers, when necessary. -->
 - [ ] The commit message is written in past tense, mentions the ticket number, and ends with a period.
 - [ ] I have checked the "Has patch" ticket flag in the Trac system.