Complete deprecation cycle for Dataset.update return value

[max-sixty]

Summary

• Completes the deprecation cycle for Dataset.update() return value that was started in v0.17
• The method now returns None instead of self, aligning with Python's standard dict.update() behavior

Changes

• Modified Dataset.update() to return None instead of self
• Updated docstring to remove return value documentation
• Added breaking change entry to changelog

Closes #10167

Test plan

• All existing tests pass without modification (they weren't using the return value)
• Verified the method still updates the dataset in-place
• Pre-commit checks pass
• Type checking passes

🤖 Generated with Claude Code