Remove 3.x references in the pull request template

.github/PULL_REQUEST_TEMPLATE.md

@@ -1,9 +1,7 @@
 <!--
 Please target the `master` branch in priority.
-PRs can target `3.x` if the same change was done in `master`, or is not relevant there.
 
 Relevant fixes are cherry-picked for stable branches as needed by maintainers.
-You can mention in the description if the change is compatible with `3.x`.
 
 To speed up the contribution process and avoid CI errors, please set up pre-commit hooks locally:
 https://docs.godotengine.org/en/latest/contributing/development/code_style_guidelines.html