Fix ConvNext V2 parameter naming issue
# What does this PR do?
Renames gamma and beta parameters of the `ConvNextV2GRN` module, which caused the `save_pretrained` method to rename these parameters to weight and bias.

Existing checkpoints on the hub can be loaded without any warnings once the PR is merged.

Fixes #23090

## Before submitting
- [ ] This PR fixes a typo or improves the docs (you can dismiss the other checks if that's the case).
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      Pull Request section?
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- [ ] Did you write any new necessary tests?


_The documentation is not available anymore as the PR was closed or merged._