[AIRFLOW-5673] Rename GcfHook to CloudFunctionsHook

[michalslowikowski00]

Part of AIP-21

Jira

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Tests

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Documentation

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[codecov-io]

Codecov Report

Merging #6414 into master will decrease coverage by 0.3%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master    #6414      +/-   ##
==========================================
- Coverage   83.96%   83.66%   -0.31%     
==========================================
  Files         627      627              
  Lines       36549    36553       +4     
==========================================
- Hits        30689    30582     -107     
- Misses       5860     5971     +111

Impacted Files	Coverage Δ
airflow/contrib/hooks/gcp_function_hook.py	100% <100%> (ø)	⬆️
airflow/gcp/hooks/functions.py	84.12% <100%> (ø)	⬆️
airflow/gcp/operators/functions.py	98.01% <100%> (ø)	⬆️
airflow/kubernetes/volume_mount.py	44.44% <0%> (-55.56%)	⬇️
airflow/kubernetes/volume.py	52.94% <0%> (-47.06%)	⬇️
airflow/kubernetes/pod_launcher.py	45.25% <0%> (-46.72%)	⬇️
airflow/kubernetes/kube_client.py	33.33% <0%> (-41.67%)	⬇️
...rflow/contrib/operators/kubernetes_pod_operator.py	70.14% <0%> (-28.36%)	⬇️
airflow/contrib/operators/ssh_operator.py	82.5% <0%> (-1.25%)	⬇️
airflow/models/taskinstance.py	93.28% <0%> (-0.51%)	⬇️
... and 1 more

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[mik-laj]

Can you also update a note in UPDATING.md file?
https://github.com/apache/airflow/blob/master/UPDATING.md#changes-to-import-paths-and-names-of-gcp-operators-and-hooks