fix: quieter logs when polling schedule changes

[aaronsteers]

Resolves meltano/meltano#6686

[simonpai]

I can confirm this fix works (in the scenario without a custom logging config file).
We can probably also leave comments like this:

list_result = subprocess.run(
    # Suppress info-level log from CLI reduce large amount of log messages.
    # If you have your own logging configuration which overrides the log level switch,
    # Set log level to WARNING on logger: meltano.cli to achieve the same effect.
    [MELTANO_BIN, "--log-level=warning", "schedule", "list", "--format=json"],

[aaronsteers]

@pandemicsyn - Apologies for the delay. I added an explanatory comment and a constant at the top of the file for easier configuration.

Merging now.