[pulsar-client-cpp] Upgrade protobuf

[kimula]

Motivation

protobuf 3.3.0 is vulnerable.
https://snyk.io/vuln/pip:protobuf

Modifications

upgrade the version of protobuf in pulsar-client-cpp to 3.20.0.

Verifying this change

• Make sure that the change passes the CI checks.

This change is a trivial rework / code cleanup without any test coverage.

Does this pull request potentially affect one of the following parts:

If yes was chosen, please highlight the changes

• Dependencies (does it add or upgrade a dependency): (no)
• The public API: (no)
• The schema: (no)
• The default values of configurations: (no)
• The wire protocol: (no)
• The rest endpoints: (no)
• The admin cli options: (no)
• Anything that affects deployment: (don't know)

Documentation

Check the box below or label this PR directly.

Need to update docs?

• no-need-doc

[BewareMyPower]

/home/runner/work/pulsar/pulsar/pulsar-client-cpp
bash: line 1: /opt/cmake/cmake-3.20.0.0-Linux-x86_64/bin/cmake: No such file or directory

Could you verify the Dockerfile locally before pushing the patch? It looks like we shouldn't modify the path to cmake.

[kimula]

@BewareMyPower I undid an unneccessary change of cmake. PTAL.