Bump protobuf to 5.26.0

[bdraco]

blockers:

• Library pins protobuf to < 5 tronikos/gassist_text#4
• needs a new https://github.com/googleapis/python-pubsub

Proposed change

changelog: protocolbuffers/protobuf@v25.1...v26.0

There is a memory leak fix for python

There are some breaking changes

[Python] Removed obsolete/duplicate setup.py from Python. (protocolbuffers/protobuf#15708) (protocolbuffers/protobuf@5722aef)
[Python] Made text_format output default to UTF-8. (protocolbuffers/protobuf@bf00034)
[Python] Remove RegisterExtension in message class (protocolbuffers/protobuf@4ebba68)
[Python] Reject extend repeated field with none iterable (Raise TypeError) (protocolbuffers/protobuf@1658213)
[Python] Remove Deprecated APIs that add non top descriptor. (protocolbuffers/protobuf@c51f111)
[Python] Check if Timestamp is valid. (protocolbuffers/protobuf@4f77929)
[Python] Remove msg.UnknownFields() support in pure python and cpp extension. (protocolbuffers/protobuf@0eac77c)
[Python] Remove deprecated syntax accessor. (protocolbuffers/protobuf@fd40c87)

Type of change

• Dependency upgrade
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• Deprecation (breaking change to happen in the future)
• Breaking change (fix/feature causing existing functionality to break)
• Code quality improvements to existing code or addition of tests

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