Add support for protobuf >= 4.21.0

[parthea]

I've noticed several generated unit tests failing when using protobuf==4.21.0. As an example, see this build log from python-tpu
.

=========================== short test summary info ============================
FAILED tests/unit/gapic/tpu_v1/test_tpu.py::test_list_nodes_pager - assert False
FAILED tests/unit/gapic/tpu_v1/test_tpu.py::test_list_nodes_async_pager - ass...
FAILED tests/unit/gapic/tpu_v1/test_tpu.py::test_list_tensor_flow_versions_pager
FAILED tests/unit/gapic/tpu_v1/test_tpu.py::test_list_tensor_flow_versions_async_pager
FAILED tests/unit/gapic/tpu_v1/test_tpu.py::test_list_accelerator_types_pager
FAILED tests/unit/gapic/tpu_v1/test_tpu.py::test_list_accelerator_types_async_pager
FAILED tests/unit/gapic/tpu_v2alpha1/test_tpu.py::test_list_nodes_pager - ass...
FAILED tests/unit/gapic/tpu_v2alpha1/test_tpu.py::test_list_nodes_async_pager
FAILED tests/unit/gapic/tpu_v2alpha1/test_tpu.py::test_list_accelerator_types_pager
FAILED tests/unit/gapic/tpu_v2alpha1/test_tpu.py::test_list_accelerator_types_async_pager
FAILED tests/unit/gapic/tpu_v2alpha1/test_tpu.py::test_list_runtime_versions_pager
FAILED tests/unit/gapic/tpu_v2alpha1/test_tpu.py::test_list_runtime_versions_async_pager

I will link 1 issue in each downstream repo so that we can track where the failures are occurring.

[parthea]

Completed