[AIRFLOW-6584] Pin cassandra driver

[potiuk]

3.21.0 release of Cassandra driver
(https://pypi.org/project/cassandra-driver/3.21.0/) broke backwards
compatibility. We need to pin it to 3.20.2

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Issue link: AIRFLOW-6584

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• Description above provides context of the change
• Commit message/PR title starts with [AIRFLOW-NNNN]. AIRFLOW-NNNN = JIRA ID^*
• Unit tests coverage for changes (not needed for documentation changes)
• Commits follow "How to write a good git commit message"
• Relevant documentation is updated including usage instructions.
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^* For document-only changes commit message can start with [AIRFLOW-XXXX].

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In case of fundamental code change, Airflow Improvement Proposal (AIP) is needed.
In case of a new dependency, check compliance with the ASF 3rd Party License Policy.
In case of backwards incompatible changes please leave a note in UPDATING.md.
Read the Pull Request Guidelines for more information.

[codecov-io]

Codecov Report

Merging #7194 into master will decrease coverage by 0.27%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master    #7194      +/-   ##
==========================================
- Coverage   85.37%   85.09%   -0.28%     
==========================================
  Files         723      723              
  Lines       39558    39558              
==========================================
- Hits        33771    33662     -109     
- Misses       5787     5896     +109

Impacted Files	Coverage Δ
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/refresh_config.py	50.98% <0%> (-23.53%)	⬇️
...rflow/contrib/operators/kubernetes_pod_operator.py	76.47% <0%> (-21.18%)	⬇️
airflow/jobs/backfill_job.py	91.59% <0%> (+0.86%)	⬆️

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