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

Summary:
`apache-airflow>2.6.3` (installed in `docs/requirements.txt`) transitively brings in `opentelemetry` which depends on `protobuf>=4.x`. This version of `protobuf` conflicts with the one required by `kfp==1.8.x` (`protobuf <= 3.20.x`).

What we oughta do is to upgrade to `kfp-2.x` but that's a much more intrusive change as we are bumping major versions. So pin to `apache-airflow==2.6.3` so that protobuf doesn't conflict with kfp.

Reviewed By: tonykao8080

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==2.10.2