Fix broken docs-build by pinning apache-airflow==2.6.3

Differential Revision: D72016216

Author: kiukchung

docs/requirements.txt

@@ -10,4 +10,6 @@ jupytext
 ipython_genutils
 # https://github.com/jupyter/nbconvert/issues/1736
 jinja2>=3.1.4
-apache-airflow
+# higher versions of airflow brings in opentelemetry which depends
+# a version of protobuf incompatible with pinned kfp in dev-requirements.txt
+apache-airflow==2.6.3