docs: Update compatibility matrix in versioning to include Lightning 2.5 series and extend PyTorch support ranges

[bhimrazy]

What does this PR do?

Fixes #20670

Updates the versioning policy documentation to:

• Add missing Lightning 2.5 series entry with PyTorch 2.1-2.7 support
• Extend PyTorch support range for Lightning 2.4 from ≤2.4 to ≤2.6

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📚 Documentation preview 📚: https://pytorch-lightning--20948.org.readthedocs.build/en/20948/