Bump celery from 5.5.3 to 5.6.3 in /backend

Bumps [celery](https://github.com/celery/celery) from 5.5.3 to 5.6.3.
- [Release notes](https://github.com/celery/celery/releases)
- [Changelog](https://github.com/celery/celery/blob/v5.6.3/Changelog.rst)
- [Commits](celery/celery@v5.5.3...v5.6.3)

---
updated-dependencies:
- dependency-name: celery
  dependency-version: 5.6.3
  dependency-type: direct:production
  update-type: version-update:semver-minor
...

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

Author: dependabot[bot]

backend/pyproject.toml

@@ -84,7 +84,7 @@ dependencies = [
     "l18n<2022.0,>=2021.3",
     "wagtail==7.2.3",
     "wagtail-localize==1.13",
-    "celery==5.5.3",
+    "celery==5.6.3",
     "wagtail-headless-preview==0.8.0",
     "Jinja2>=3.1.3",
     "icalendar>=5.0.11",