Bump myst-parser from 3.0.1 to 4.0.0 in /requirements (#22543)

* Bump myst-parser from 3.0.1 to 4.0.0 in /requirements

Bumps [myst-parser](https://github.com/executablebooks/MyST-Parser) from 3.0.1 to 4.0.0.
- [Release notes](https://github.com/executablebooks/MyST-Parser/releases)
- [Changelog](https://github.com/executablebooks/MyST-Parser/blob/master/CHANGELOG.md)
- [Commits](https://github.com/executablebooks/MyST-Parser/compare/v3.0.1...v4.0.0)

---
updated-dependencies:
- dependency-name: myst-parser
  dependency-type: direct:development
  update-type: version-update:semver-major
...

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

* TMP: Fix bug with header ordering.

---------

Signed-off-by: dependabot[bot] <support@github.com>
Co-authored-by: dependabot[bot] <49699333+dependabot[bot]@users.noreply.github.com>
Co-authored-by: Kevin Meinhardt <kmeinhardt@mozilla.com>

README.md

@@ -17,7 +17,7 @@ You can access the AMO dev environment at <https://addons-dev.allizom.org/> and
 
 % marker-for-security-bug-inclusion-do-not-remove
 
-## Security Bug Reports
+# Security Bug Reports
 
 This code and its associated production web page are included in the Mozilla’s web and services [bug bounty program]. If you find a security vulnerability, please submit it via the process outlined in the program and [FAQ pages]. Further technical details about this application are available from the [Bug Bounty Onramp page].
 

requirements/dev.txt

@@ -89,9 +89,9 @@ sphinx-rtd-theme==2.0.0 \
 snowballstemmer==2.2.0 \
     --hash=sha256:c8e1716e83cc398ae16824e5572ae04e0d9fc2c6b985fb0f900f5f0c96ecba1a \
     --hash=sha256:09b16deb8547d3412ad7b590689584cd0fe25ec8db3be37788be3810cbf19cb1
-myst-parser==3.0.1 \
-    --hash=sha256:6457aaa33a5d474aca678b8ead9b3dc298e89c68e67012e73146ea6fd54babf1 \
-    --hash=sha256:88f0cb406cb363b077d176b51c476f62d60604d68a8dcdf4832e080441301a87
+myst-parser==4.0.0 \
+    --hash=sha256:851c9dfb44e36e56d15d05e72f02b80da21a9e0d07cba96baf5e2d476bb91531 \
+    --hash=sha256:b9317997552424448c6096c2558872fdb6f81d3ecb3a40ce84a7518798f3f28d
 markdown-it-py==3.0.0 \
     --hash=sha256:355216845c60bd96232cd8d8c40e8f9765cc86f46880e43a8fd22dc1a1a8cab1 \
     --hash=sha256:e3f60a94fa066dc52ec76661e37c851cb232d92f9886b15cb560aaada2df8feb