[AIRFLOW-6209] Drop gcp_service_account_keys option

[mik-laj]

I wanna remove this option, because it is no longer supported by the Google Cloud Platform. The new
recommended service account keys management method for the GCP platform is
Workload Identity.

After merge, we should also add note in https://cwiki.apache.org/confluence/display/AIRFLOW/Managing+Per-task+Credentials+in+KubernetesExecutor

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[codecov-io]

Codecov Report

Merging #6768 into master will decrease coverage by 0.28%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master    #6768      +/-   ##
==========================================
- Coverage   84.54%   84.26%   -0.29%     
==========================================
  Files         672      672              
  Lines       38081    38072       -9     
==========================================
- Hits        32197    32081     -116     
- Misses       5884     5991     +107

Impacted Files	Coverage Δ
airflow/executors/kubernetes_executor.py	57.92% <ø> (-0.25%)	⬇️
airflow/kubernetes/pod_generator.py	95.27% <ø> (+0.61%)	⬆️
airflow/operators/postgres_operator.py	0% <0%> (-100%)	⬇️
airflow/executors/sequential_executor.py	56% <0%> (-32%)	⬇️
airflow/utils/sqlalchemy.py	88.13% <0%> (-8.48%)	⬇️
airflow/utils/dag_processing.py	80.34% <0%> (-7.45%)	⬇️
airflow/executors/executor_loader.py	56.52% <0%> (-6.53%)	⬇️
airflow/jobs/scheduler_job.py	87.77% <0%> (-1.79%)	⬇️
airflow/hooks/dbapi_hook.py	89.83% <0%> (-1.7%)	⬇️
airflow/hooks/postgres_hook.py	92.95% <0%> (-1.41%)	⬇️
... and 5 more

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