[Github] Update pull_request_template.md (#14723)

### Change Description

Make the PR template a little easier to work with.

### Security Assessment

- This change has no security impact

Description of the security impact and necessary mitigations:

No impact because this is not code being deployed to production

(Reviewers: please confirm the security impact before approving)

.github/pull_request_template.md

@@ -1,20 +1,23 @@
-### Change Description
+## Change Description
 
-Fixes #<issue_number>.
+Fixes #<issue_number> (delete if N/A)
 
-### Security Assessment
+Brief description and justification of what this PR is doing.
 
-- [ ] This change has a high security impact
-  - [ ] Required: and the impact has been assessed and approved by appsec
-- [ ] This change has a medium security impact
-- [ ] This change has a low security impact
-- [ ] This change has no security impact
+## Security Assessment
 
-Description of the security impact and necessary mitigations:
+Delete all except the correct answer:
+- This change has a high security impact
+  - [ ] Required: The impact has been assessed and approved by appsec
+- This change has a medium security impact
+- This change has a low security impact
+- This change has no security impact
 
-- For none/low impact: a quick one/two sentence justification of the rating.
+### Impact Description
+
+For none/low impact: a quick one/two sentence justification of the rating.
   - Example: "Docs only", "Low-level refactoring of non-security code", etc.
-- For medium/high impact: provide a description of the impact and the mitigations in place.
+For medium/high impact: provide a description of the impact and the mitigations in place.
   - Example: "New UI text field added in analogy to existing elements, with input strings escaped and validated against code injection"
 
-(Reviewers: please confirm the security impact before approving)
+(Reviewers: please confirm the security impact before approving)