Bump the rev of protobuf supported to 4.21.8

[nipunn1313]

Bumping rev of mypy-protobuf incoming next

This change is fairly minor - just some comment changes in the generated stubs.

[AlexWaygood]

We were wondering about removing google/__init__.pyi in our next major version bump for our protobuf stubs. Would you be interested in making that change as part of this PR? And if so -- would you be okay if we held off merging this until mypy 0.990 is released (should be in the next few weeks)? --namespace-packages will be the default in mypy 0.990, so it will make removing google/__init__.pyi much less painful for users (but we'd still rather make the change along with a major version bump, just to be sure).

[nipunn1313]

Sure that works.

One idea could be to keep this at 3.21.8 until then - just to unblock/decouple this work from the __init__ work - since this piece is not actually a breaking change.

[github-actions]

According to mypy_primer, this change has no effect on the checked open source code. 🤖🎉