MNT: (deps): Bump myst-parser from 0.18.1 to 4.0.0 in /ci

Bumps [myst-parser](https://github.com/executablebooks/MyST-Parser) from 0.18.1 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](executablebooks/MyST-Parser@v0.18.1...v4.0.0)

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

Signed-off-by: dependabot[bot] <[email protected]>

ci/doc_requirements.txt

@@ -4,5 +4,5 @@ metpy==1.6.3
 sphinx==5.2.3
 sphinx_rtd_theme==1.0.0
 sphinx-gallery==0.18.0
-myst-parser==0.18.1
+myst-parser==4.0.0
 netCDF4==1.7.2