Update proto stubs with latest mypy-protobuf 2.6

[nipunn1313]

Updated protobuf version and mypy-protobuf version for newly updated stubs.
All updated stubs were autogenerated by scripts/generate_proto_stubs.sh

[srittau]

Could you please also update the version in the METADATA.toml file?

[Akuli]

Should we cherry-pick changes that were made manually?

[nipunn1313]

Could you please also update the version in the METADATA.toml file?

Yep! on it

Should we cherry-pick changes that were made manually?

Let me look through them. I think there's no need.

git log -p HEAD^ -- stubs/protobuf/**/*_pb2.pyi
git log -p -- "third_party/2and3/google/protobuf/**/*_pb2.pyi"

#5597 - looks like pyright is passing here. The improved mypy-protobuf seems to have made these changes.
#4971 - just reorganized dir structure for stubs
#5252 - ran black, but the generation script already runs black - and tests pass here, so no problem
#4850 - was the last update of mypy-protobuf pin

[nipunn1313]

ah - my git command missed things. Trying again with

git log -p -- "third_party/2and3/google/**/*_pb2.pyi"

Also came across
#4907 - which has since been fixed upstream in mypy-protobuf

That should be comprehensive audit of the cherry-picks. I don't think they're needed any more (all the better!)