Update PR Template (#25039)

## Summary & Motivation

Simplifies the PR template and removes the categorization system, as it
adds friction and post-hoc categorization is not particularly difficult.

## How I Tested These Changes

.github/PULL_REQUEST_TEMPLATE.md

@@ -4,8 +4,4 @@
 
 ## Changelog
 
-Insert changelog entry or "NOCHANGELOG" here.
-
-- [ ] `NEW` _(added new feature or capability)_
-- [ ] `BUGFIX` _(fixed a bug)_
-- [ ] `DOCS` _(added or updated documentation)_
+> Insert changelog entry or delete this section.