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>

Author: dependabot[bot]

requirements.txt

@@ -51,7 +51,7 @@ urllib3==1.24.1
 virtualenv==16.1.0
 virtualenv-clone==0.5.1
 virtualenvwrapper-win==1.2.5
-wagtail==2.7
+wagtail==2.11.8
 webencodings==0.5.1
 whitenoise==4.1.2
 Willow==1.3