[noissue]: Update mistune requirement from <2.0.0 to <4.0.0

Updates the requirements on [mistune](https://github.com/lepture/mistune) to permit the latest version.
- [Release notes](https://github.com/lepture/mistune/releases)
- [Changelog](https://github.com/lepture/mistune/blob/master/docs/changes.rst)
- [Commits](lepture/mistune@v0.1.0...v3.0.0)

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

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

doc_requirements.txt

@@ -5,5 +5,5 @@ sphinx-rtd-theme==1.2.0
 sphinxcontrib-jquery
 sphinxcontrib-openapi
 towncrier
-mistune<2.0.0
+mistune<4.0.0
 Jinja2<3.2