[bugfix] fixup warning message for plugged schedulers for v1

[joerunde]

Quick fix for a small bug with the log introduced in #14466

That warning log always prints, since we actually configure the v1 scheduler as a string. 🤦

This change instead checks the resolved class, using an equality check against the existing V1 scheduler class. I did it this way, instead of isinstance(scheduler, V1Scheduler), since existing scheduler classes may inherit from the V1 scheduler and we still want to print the warning if that's the case.

[github-actions]

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