Support Protobuf 6.x.

[yilei]

Why? Currently Apache Beam pins protobuf<6.0.0.dev0. Protobuf 6.30.0 was released on March 4, 2025: https://pypi.org/project/protobuf/6.30.0/, https://github.com/protocolbuffers/protobuf/releases/tag/v30.0 is the corresponding release.

Also fixed two flaky tests that became more flaky with the new Protobuf version. They were flaky because Protobuf doesn't guarantee map iteration order, so the sampler instances created here doesn't guarantee 'a' is the first and 'b' is the second sampler when the tests use self.make_test_descriptor(outputs=['a', 'b']).

Fixes #35432.

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[github-actions]

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R: @damccorm for label python.

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[damccorm]

Thanks - this generally LGTM. @jrmccluskey would you mind taking a look? (any useful context I might be missing from the last protobuf upgrade #32679 ?)

[tvalentyn]

unless there is some urgency, I would do it after the release cut, update dev containers, and test with internal user base to have more time to detect potential performance regressions. protobuf upgrades are hit-or-miss in my experience.

[github-actions]

Reminder, please take a look at this pr: @damccorm

[damccorm]

Merging since the cut has passed