Update pull request template for website freeze (flutter#8041)

.github/PULL_REQUEST_TEMPLATE.md

@@ -1,8 +1,5 @@
-_Description of what this PR is changing or adding, and why:_
+**IMPORTANT:** Due to work on the docs.flutter.dev infrastructure, this repo is **not accepting pull requests**.
 
-_Issues fixed by this PR (if any):_ Fixes #ISSUE-NUMBER
+Instead of creating a PR, please file an issue (https://github.com/flutter/website/issues/new/choose) about the needed change.
 
-## Presubmit checklist
-- [ ] This PR doesn’t contain automatically generated corrections (Grammarly or similar).
-- [ ] This PR follows the [Google Developer Documentation Style Guidelines](https://developers.google.com/style) — for example, it doesn’t use _i.e._ or _e.g._, and it avoids _I_ and _we_ (first person).
-- [ ] This PR uses [semantic line breaks](https://github.com/dart-lang/site-shared/blob/main/doc/writing-for-dart-and-flutter-websites.md#semantic-line-breaks) of 80 characters or fewer.
+We expect to start accepting PRs again the week of January 24th.