Python: Downgrade protobuf dependency to >=3.20

[liorsve]

Summary

• Lowers the minimum protobuf Python dependency from >=6.20 to >=3.20 for glide-async and glide-shared
• Installs protoc 3.20.0 in the PyPI CD workflow to generate backward-compatible protobuf files
• Broadens compatibility for users pinned to older protobuf versions

issue - #5221

Tested Compatibility Matrix

Validated on the PyPI CD workflow across 4 platforms (Linux x86_64, Linux aarch64, macOS arm64, macOS x86_64) with both async and sync clients.

CPython

Python	3.20.0	3.20.3	4.21.12	4.25.8	5.26.1	5.29.6	6.30.2	6.33.5
3.9	✅	✅	✅	✅	✅	✅	✅	✅
3.10	✅	✅	✅	✅	✅	✅	✅	✅
3.11	✅	✅	✅	✅	✅	✅	✅	✅
3.12	✅	✅	✅	✅	✅	✅	✅	✅
3.13	✅	✅	✅	✅	✅	✅	✅	✅
3.14	⏭️	⏭️	⏭️	⏭️	⏭️	✅	✅	✅

PyPy

PyPy	3.20.0	3.20.3	4.21.12	4.25.8	5.26.1	5.29.6	6.30.2	6.33.5
3.9	✅	✅	✅	✅	✅	✅	✅	✅
3.10	✅	✅	✅	✅	✅	✅	✅	✅
3.11	✅	✅	✅	✅	✅	✅	✅	✅

⏭️ = Skipped (Python 3.14 + protobuf < 5.29 — known incompatibility with metaclass tp_new removal)