Revert "[bots] remove auto-assign for web PRs (flutter#15878)"

This reverts commit a4cdd72.

Author: NoamDev

.github/auto_assign.yml

@@ -38,4 +38,3 @@ numberOfReviewers: 1
 # A list of keywords to be skipped the process that add reviewers if pull requests include it
 skipKeywords:
   - Roll
-  - web