[FLINK-37482] Improve the code for PrintSinkOutputWriter, PrintSinkFunction and PrintSink

[beliefer]

What is the purpose of the change

This PR aim to improve the code for PrintSinkOutputWriter, PrintSinkFunction and PrintSink.

1. Rename the variable target with a meaning name.
2. Simplify the code like stream = target == STD_OUT ? System.out : System.err;.

Brief change log

Simplify the code for PrintSink

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, Kubernetes/Yarn, ZooKeeper: (no)
• The S3 file system connector: (no)

Documentation

• Does this pull request introduce a new feature? (no)

[flinkbot]

CI report:

• 1806923 Azure: SUCCESS

Bot commands

The @flinkbot bot supports the following commands:

• @flinkbot run azure re-run the last Azure build

[beliefer]

ping @zentol @1996fanrui @davidradl cc @ifndef-SleePy

[1996fanrui]

Thanks @beliefer for the refactor!

Overall LGTM, I only left one comment. Please take a look in your free time, thanks~

[1996fanrui]

Suggested change

	private static final boolean STD_OUT = false;
	private static final boolean STD_ERR = true;

Could these 2 lines be removed after refactoring the target to isStdErr?

[beliefer]

Good thinking.