[AIRFLOW-5479] Normalize gcp_conn method in GCP Kubernetes Hook

[TobKed]

Make sure you have checked all steps below.

Jira

• My PR addresses the following Airflow Jira issues and references them in the PR title. For example, "[AIRFLOW-XXX] My Airflow PR"
  • https://issues.apache.org/jira/browse/AIRFLOW-5479
  • In case you are fixing a typo in the documentation you can prepend your commit with [AIRFLOW-XXX], code changes always need a Jira issue.
  • In case you are proposing a fundamental code change, you need to create an Airflow Improvement Proposal (AIP).
  • In case you are adding a dependency, check if the license complies with the ASF 3rd Party License Policy.

Description

• Here are some details about my PR, including screenshots of any UI changes:

Tests

• My PR adds the following unit tests OR does not need testing for this extremely good reason:

Commits

• My commits all reference Jira issues in their subject lines, and I have squashed multiple commits if they address the same issue. In addition, my commits follow the guidelines from "How to write a good git commit message":
  1. Subject is separated from body by a blank line
  2. Subject is limited to 50 characters (not including Jira issue reference)
  3. Subject does not end with a period
  4. Subject uses the imperative mood ("add", not "adding")
  5. Body wraps at 72 characters
  6. Body explains "what" and "why", not "how"

Documentation

• In case of new functionality, my PR adds documentation that describes how to use it.
  • All the public functions and the classes in the PR contain docstrings that explain what it does
  • If you implement backwards incompatible changes, please leave a note in the Updating.md so we can assign it to a appropriate release

[mik-laj]

Can you add backward compatibility and deprecation warning?

[TobKed]

Sure. I've added it. Please check is it correct.

[codecov-io]

Codecov Report

Merging #6099 into master will decrease coverage by 0.02%.
The diff coverage is 77.77%.

@@            Coverage Diff             @@
##           master    #6099      +/-   ##
==========================================
- Coverage    80.1%   80.08%   -0.03%     
==========================================
  Files         606      606              
  Lines       34893    34897       +4     
==========================================
- Hits        27951    27947       -4     
- Misses       6942     6950       +8

Impacted Files	Coverage Δ
airflow/gcp/hooks/kubernetes_engine.py	95.89% <77.77%> (-2.67%)	⬇️
airflow/models/taskinstance.py	93.22% <0%> (-0.51%)	⬇️
airflow/jobs/scheduler_job.py	74.28% <0%> (-0.31%)	⬇️
airflow/utils/dag_processing.py	58.8% <0%> (-0.19%)	⬇️

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update 0033751...c77fee6. Read the comment docs.