fix: celery issue with eta/countdown affecting grading

There was a bug in celery 5.6.1 that caused having eta/countdown
parameters being stuck in received and never executing until we
downgraded to 5.6.0. This issue appears to be been fixed in 5.6.2.

Author: ormsbee

requirements/constraints.txt

@@ -17,11 +17,13 @@
 # Stay on LTS version, remove once this is added to common constraint
 Django<6.0
 
-# Date: 2020-02-26
-# As it is not clarified what exact breaking changes will be introduced as per
-# the next major release, ensure the installed version is within boundaries.
+# Date: 2026-01-13
+# We would normally pin celery to <6.0.0 to avoid auto-updating across a major
+# version boundary without more thorough testing. The reason it's currently also
+# pinned to !=5.6.1 is because of a celery bug related to the eta and countdown
+# parameters. This bug caused operational issues in MIT's deployment.
 # Issue for unpinning: https://github.com/openedx/edx-platform/issues/35280
-celery>=5.2.2,<6.0.0
+celery>=5.2.2,!=5.6.1,<6.0.0
 
 # Date: 2020-02-10
 # django-oauth-toolkit version >=2.0.0 has breaking changes. More details