Bump wagtail from 2.9 to 2.11.8

Bumps [wagtail](https://github.com/wagtail/wagtail) from 2.9 to 2.11.8.
- [Release notes](https://github.com/wagtail/wagtail/releases)
- [Changelog](https://github.com/wagtail/wagtail/blob/main/CHANGELOG.txt)
- [Commits](wagtail/wagtail@v2.9...v2.11.8)

---
updated-dependencies:
- dependency-name: wagtail
  dependency-type: direct:production
...

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

requirements.txt

@@ -31,6 +31,6 @@ python-ffmpeg-video-streaming==0.1.5
 celery==4.4.2
 # install redis-server on server
 redis==3.5.2
-wagtail==2.9
+wagtail==2.11.8
 drf-yasg==1.17.1
 django-environ==0.4.5