[release-2.27.0][BEAM-11524] Fix WriteStringsToPubSub, remove usages in examples

[TheNeuralBit]

This cherrypicks three PRs into the 2.27.0 branch. #13614 and #13615 for BEAM-11524, and #13623 (restrict pandas version to <1.2.0 for BEAM-11531).

R: @pabloem
CC: @boyuanzz

Post-Commit Tests Status (on master branch)

Lang	SDK	Dataflow	Flink	Samza	Spark	Twister2
Go		---		---		---
Java
Python				---		---
XLang				---		---

Pre-Commit Tests Status (on master branch)

---	Java	Python	Go	Website	Whitespace	Typescript
Non-portable
Portable	---		---	---	---	---

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GitHub Actions Tests Status (on master branch)

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[codecov]

Codecov Report

Merging #13627 (c699127) into release-2.27.0 (391ea8e) will decrease coverage by 0.00%.
The diff coverage is 100.00%.

@@                Coverage Diff                 @@
##           release-2.27.0   #13627      +/-   ##
==================================================
- Coverage           82.70%   82.69%   -0.01%     
==================================================
  Files                 466      466              
  Lines               57514    57510       -4     
==================================================
- Hits                47566    47560       -6     
- Misses               9948     9950       +2     

Impacted Files	Coverage Δ
sdks/python/apache_beam/examples/sql_taxi.py	0.00% <ø> (ø)
...s/python/apache_beam/examples/snippets/snippets.py	76.97% <100.00%> (ø)
sdks/python/apache_beam/io/gcp/pubsub.py	92.19% <100.00%> (-0.15%)	⬇️
...python/apache_beam/runners/direct/direct_runner.py	93.85% <100.00%> (ø)
sdks/python/apache_beam/io/iobase.py	84.81% <0.00%> (-0.27%)	⬇️
...ks/python/apache_beam/runners/worker/sdk_worker.py	89.69% <0.00%> (-0.16%)	⬇️
setup.py	0.00% <0.00%> (ø)

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