Bump sphinx-markdown-builder from 0.5.5 to 0.6.0 in /third_party/airflow

Bumps [sphinx-markdown-builder](https://github.com/liran-funaro/sphinx-markdown-builder) from 0.5.5 to 0.6.0.
- [Commits](https://github.com/liran-funaro/sphinx-markdown-builder/commits/0.6.0)

---
updated-dependencies:
- dependency-name: sphinx-markdown-builder
  dependency-type: direct:production
  update-type: version-update:semver-minor
...

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

third_party/airflow/pyproject.toml

@@ -22,7 +22,7 @@ readme = "README.md"
 format = ["black==23.3.0", "flake8==6.0.0", "pylint==2.17.4"]
 test = ["pytest==7.3.1", "coverage>=6.5.0", "pytest-asyncio==0.21.0"]
 # note(JayF): sphinx-jekyll-builder was broken by sphinx-markdown-builder 0.6 -- so pin to 0.5.5
-docs = ["sphinx==7.0.1", "sphinx-jekyll-builder==0.3.0", "sphinx-toolbox==3.2.0b1", "sphinx-markdown-builder==0.5.5"]
+docs = ["sphinx==7.0.1", "sphinx-jekyll-builder==0.3.0", "sphinx-toolbox==3.2.0b1", "sphinx-markdown-builder==0.6.0"]
 
 [build-system]
 requires = ["setuptools"]