[Image processors] Fix warnings
# What does this PR do?

This PR aims to reduce the number of warnings shown to the user for 2 use cases:

- DETR and friends' `max_size` argument which is deprecated => if this gets approved, I'll run fix-copies to fix the other DETR-based models
- the pattern matching warning when a configuration doesn't have an image processor in the config
_The documentation is not available anymore as the PR was closed or merged._<|||||>Thanks for the review! After offline discussion, will close this PR in favor of a new one that just updates the warning levels. The removal of `max_size` can be done in a follow-up PR.