Update versioning governance document

[speediedan]

What does this PR do?

This versioning governance document update makes the following clarifications and minor updates:

1. Clarifies the existing Lightning API evolution policy
2. Makes the Compatiblity Matrix discussion more explicit with respect to expected PyTorch support ranges (while retaining flexibility)
3. Bumps current 2.5.x support range to include PyTorch 2.8.0

Updates to the versioning governance revolve around the discussion in #21073 and should resolve that issue.

Fixes #21073

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📚 Documentation preview 📚: https://pytorch-lightning--21107.org.readthedocs.build/en/21107/

[Copilot]

Pull Request Overview

This PR updates the versioning governance document to clarify Lightning's API evolution policy and compatibility matrix. The changes simplify the API deprecation policy and make PyTorch support expectations more explicit.

Key changes:

• Simplifies API evolution policy to limit breaking changes to major version upgrades only
• Updates compatibility matrix to clarify PyTorch support expectations (latest 5 minor releases)
• Updates PyTorch 2.5.x support range to include version 2.8.0

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