Bump celery from 5.5.3 to 5.6.3 in /backend (#4615)

Co-authored-by: dependabot[bot] <49699333+dependabot[bot]@users.noreply.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",