Bump moto from 5.0.17 to 5.0.21 in /open-social

Bumps [moto](https://github.com/getmoto/moto) from 5.0.17 to 5.0.21.
- [Release notes](https://github.com/getmoto/moto/releases)
- [Changelog](https://github.com/getmoto/moto/blob/master/CHANGELOG.md)
- [Commits](getmoto/moto@5.0.17...5.0.21)

---
updated-dependencies:
- dependency-name: moto
  dependency-type: direct:production
  update-type: version-update:semver-patch
...

Signed-off-by: dependabot[bot] <support@github.com>

open-social/tests/test_requirements.txt

@@ -2,6 +2,6 @@
 -r ../open_social_core/requirements.txt
 freezegun==1.5.1
 mongomock==4.2.0.post1
-moto==5.0.17
+moto==5.0.21
 pytest==8.3.3
 pyyaml==6.0.2