[FLINK-11512] Port CliFrontendModifyTest to new code base

[tisonkun]

What is the purpose of the change

Replace StandaloneClusterClient with RestClusterClient<StandaloneClusterId>

Verifying this change

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

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

• Dependencies (does it add or upgrade a dependency): (no)
• The public API, i.e., is any changed class annotated with @Public(Evolving): (no)
• The serializers: (no)
• The runtime per-record code paths (performance sensitive): (no)
• Anything that affects deployment or recovery: JobManager (and its components), Checkpointing, Yarn/Mesos, ZooKeeper: (no)
• The S3 file system connector:(no)

Documentation

• Does this pull request introduce a new feature? (no)
• If yes, how is the feature documented? (not applicable)

cc @tillrohrmann @mxm

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[mxm]

Does this remove test coverage for the old client?

[tisonkun]

Stephan Ewen on ML mentioned that we will finally use a HTTP client. Thus we can only check RestClusteClient.

Also FLINK-11540 intends to remove StandaloneClusterClient since it is legacy now.

[mxm]

Thanks, makes sense.