Bump wagtail from 2.7 to 2.11.8

Bumps [wagtail](https://github.com/wagtail/wagtail) from 2.7 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@v2.7...v2.11.8)

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

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

requirements.txt

@@ -1,5 +1,5 @@
 # Minimal requirements
-wagtail==2.7
+wagtail==2.11.8
 django-taggit==1.0
 Django==2.2
 django-modeltranslation==0.14