Use airflow released in PyPI for k8s test environment (#35099)

So far we've been using ".[cncf.kubernetes]" as requirement for
creating kubernetes test virtualenve (in k8s_requirements.txt).

This becomes problematic when we add a new pre-installed provider
that is not yet released in PyPI - because it cannot be found
as we found out in #82689.

Changing it to "apache-airflow[cncf.kubernetes]" makes it install
airflow + k8s requirements from PyPI instead.

GitOrigin-RevId: dc2e8522bfb377daf6e2e9cf19b265f13dfe41c1

scripts/ci/kubernetes/k8s_requirements.txt

@@ -1,4 +1,4 @@
-.[cncf.kubernetes]
+apache-airflow[cncf.kubernetes]
 pytest
 pytest-cov
 pytest-instafail