Convert validation loop config warnings to PossibleUserWarning

[CompRhys]

What does this PR do?

Extends #10644 by making more warnings PossibleUserWarning, related to #13358 for cleaning-up/reducing stdout verbosity.

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

If users are already filtering UserWarning code will no longer filter messages converted to PossibleUserWarning.

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[CompRhys]

Have updated CHANGELOG.md but feel this is minor