Raise protobuf minimum version in Python requirements

[anzzraju1997-glitch]

Changes

• Update requirements.txt to protobuf>=4.25.8
• Update requirements-training.txt to protobuf>=4.25.8
• Update requirements-dev.txt to protobuf>=4.25.8
• Update docs/python/requirements.txt to protobuf>=4.25.8

Notes

This change addresses the direct Python manifest surface only. It does not claim to resolve every transitive Component Governance finding.

[anzzraju1997-glitch]

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