[AIRFLOW-2888] Add deprecation path for task_runner config change

[ashb]

Make sure you have checked all steps below.

Jira

https://issues.apache.org/jira/browse/AIRFLOW-2888 (extending the change introduced in this Jira to provide a deprecation/upgrade path.)

Description

This change allows users to seamlessly upgrade without a hard-to-debug
error when a task is actually run. This allows us to pull the change introduced in #3740 to 1.10.3 whilst still being nice to our users.

Tests

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

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.
  • When adding new operators/hooks/sensors, the autoclass documentation generation needs to be added.
  • 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

Code Quality

• Passes flake8

[ashb]

cc @Fokko @feng-tao I'd like to get this change in so I can cherry pick the main change from this Jira to 1.10.3 (because that change is a dependency for some of Kevin Yang of AirBNB's awesome celery performance changes.)

[codecov-io]

Codecov Report

Merging #4851 into master will increase coverage by 0.03%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master    #4851      +/-   ##
==========================================
+ Coverage   74.71%   74.74%   +0.03%     
==========================================
  Files         449      449              
  Lines       28961    28973      +12     
==========================================
+ Hits        21638    21656      +18     
+ Misses       7323     7317       -6

Impacted Files	Coverage Δ
airflow/configuration.py	94.4% <100%> (+1.33%)	⬆️
airflow/models/__init__.py	92.6% <0%> (-0.06%)	⬇️
airflow/contrib/kubernetes/kube_client.py	75% <0%> (+16.66%)	⬆️

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