Allow backbones not in backbones_supported - Maskformer Mask2Former
# What does this PR do?

Updates configuration creation to allow backbone configs to be pased in which aren't listed in `backbones_supported` - downgrading the exception raised to a warning. 

Tested with:

```python
from transformers import (
    Mask2FormerConfig, 
    MaskFormerConfig, 
    Mask2FormerModel, 
    MaskFormerModel, 
    FocalNetConfig
)

# Test with a backbone not officially supported
backbone_config = FocalNetConfig(out_indices=(-2, -1))

maskformer_config = MaskFormerConfig(backbone_config=backbone_config)
model = MaskFormerModel(maskformer_config)

mask2former_config = Mask2FormerConfig(backbone_config=backbone_config)
model = Mask2FormerModel(mask2former_config)
```

Fixes #24244


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