Update third_party/protobuf to 3.21.12

[jtattermusch]

Change was created by the release automation script. See go/grpc-release

• a few manual fixes have been added.
• Even though "protobuf@master" is currently broken, it's probably a good idea to upgrade protobuf to the latest released version in order to minimize the diff.

[jtattermusch]

Python311 artifact build on windows failure is interesting:
https://source.cloud.google.com/results/invocations/435291ee-873b-4ca3-b8bd-94841beacf56/targets/github%2Fgrpc%2Fbuild_artifacts_python/tests;passed=true;group=tasks;test=build_artifact.python_windows_x64_Python311;row=4

[jtattermusch]

The sanity test error seem to be due to a design flaw in check_protobuf_pod_version.sh which, takes version1=$(git describe --tags | cut -f 1 -d'-') - which currently evaluates to v21.12 due to the change in protobuf versioning scheme.

+ '[' 21.12 '!=' 3.21.12 ']'
+ echo 'Protobuf version in src/objective-c/!ProtoCompiler.podspec does not match protobuf version in third_party/protobuf.'
Protobuf version in src/objective-c/!ProtoCompiler.podspec does not match protobuf version in third_party/protobuf.

[jtattermusch]

Python311 artifact build on windows failure is interesting: https://source.cloud.google.com/results/invocations/435291ee-873b-4ca3-b8bd-94841beacf56/targets/github%2Fgrpc%2Fbuild_artifacts_python/tests;passed=true;group=tasks;test=build_artifact.python_windows_x64_Python311;row=4

Details

And it might have been a one-off flake.

[jtattermusch]

CC @sampajano for the check_protobuf_pod_version.sh fix.

[sampajano]

CC @sampajano for the check_protobuf_pod_version.sh fix.

Thanks for cc! I'm not very familiar with protobuf versioning but i suppose this "3.x.y" format won't work forever.. hence i've added a comment for you to link to the source of why you're making this change so we can update it in the future when appropriate. Thanks!

[sampajano]

Thanks a lot for the change! iOS related change LGTM thanks!