[BEAM-8932][Cleanup] Change default PubsubIO client from json to grpc.

[dpcollins-google]

Diffbased on #10478

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Post-Commit Tests Status (on master branch)

Lang	SDK	Apex	Dataflow	Flink	Gearpump	Samza	Spark
Go		---	---		---	---
Java
Python		---			---	---
XLang	---	---	---		---	---	---

Pre-Commit Tests Status (on master branch)

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

See .test-infra/jenkins/README for trigger phrase, status and link of all Jenkins jobs.

[dpcollins-google]

+R: @udim

[aaltay]

R: @chamikaramj could you find a person to review this PR?

[lukecwik]

Note there is another contributor working on updating grpc from 1.17 to 1.26 in #10438 which may impact this change.

You might want to use the linkage checker to validate the upgrade.

[chamikaramj]

R: @udim

Udi, can you take a look since you are more familiar regarding PubSub.

[udim]

retest this please

[udim]

Looks like this is PR is no. 4 in a chain of PRs (none yet merged). I'll wait for those to be merged before reviewing, so that the diff is smaller.

[udim]

retest this please

[stale]

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[aaltay]

I believe this PR is still active.

[stale]

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[stale]

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