Bump celery in /{{cookiecutter.project_slug}}/backend (#130)

Bumps [celery](https://github.com/celery/celery) from 4.4.7 to 5.0.0.
- [Release notes](https://github.com/celery/celery/releases)
- [Changelog](https://github.com/celery/celery/blob/master/Changelog.rst)
- [Commits](celery/celery@v4.4.7...v5.0.0)

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Co-authored-by: dependabot-preview[bot] <27856297+dependabot-preview[bot]@users.noreply.github.com>

Author: dependabot-preview[bot]

{{cookiecutter.project_slug}}/backend/requirements.txt

@@ -1,7 +1,7 @@
 alembic==1.4.2
 Authlib==0.14.3
 fastapi==0.61.1
-celery==4.4.7
+celery==5.0.0
 redis==3.5.3
 httpx==0.14.2
 ipython==7.18.1