Fix trainer.logger deprecation message

[carmocca]

What does this PR do?

Fixes slack report:

I’m passing in multiple loggers to the trainer and getting spammed with
UserWarning: Using trainer.logger when Trainer is configured to use multiple loggers. This behavior will change in v1.8 when LoggerCollection is removed, and trainer.logger will return the first logger in trainer.loggers
Is there something I need to be doing differently to avoid this warning?

Does your PR introduce any breaking changes? If yes, please list them.

None

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cc @awaelchli @edward-io @Borda @ananthsub @rohitgr7 @kamil-kaczmarek @Raalsky @Blaizzy