templates: Improve review management comments.

- The new language is more brief.
- There is an option for the contributor to say that they won't
  be updating their PR.

Author: alya

config/templates/needsReviewWarning.md

@@ -1,3 +1 @@
-Hello @{assignees}, @{author} updated this pull request over {days} days ago in response to your review, but you haven't checked back in on their changes! Please leave some feedback for them or mark this pull request as ready for merging.
-
-Thank you for reviewing this pull request!
+Hello @{assignees}, @{author} updated this pull request over {days} days ago in response to your review. Please leave some feedback on next steps for this PR. Thanks!

config/templates/updateWarning.md

@@ -1,5 +1,3 @@
-Hello @{author}, a Zulip maintainer reviewed this pull request over {days} days ago, but you haven't updated your pull request since. Please take a look at the requested changes and update your pull request accordingly.
-
-Thank you for your valuable contributions to Zulip!
+@{author} A Zulip maintainer reviewed this PR over {days} days ago. Please take a look at the requested changes and update your PR, or leave a comment if you will not be able to do so soon. Thanks!
 
 <!-- updateWarning -->