Add support for protobuf 5.x

[parthea]

Protobuf 5.x is not currently supported in generated clients.

See blocking issue googleapis/proto-plus-python#431

gapic-generator-python/setup.py

Lines 27 to 35 in 24a23a1

	dependencies = [
	# Ensure that the lower bounds of these dependencies match what we have in the
	# templated setup.py.j2: https://github.com/googleapis/gapic-generator-python/blob/main/gapic/templates/setup.py.j2
	"click >= 6.7",
	"google-api-core[grpc] >= 1.34.1, <3.0.0dev,!=2.0.*,!=2.1.*,!=2.2.*,!=2.3.*,!=2.4.*,!=2.5.*,!=2.6.*,!=2.7.*,!=2.8.*,!=2.9.*,!=2.10.*",
	"googleapis-common-protos >= 1.55.0",
	"grpcio >= 1.24.3",
	"jinja2 >= 2.10",
	"protobuf>=3.19.5,<5.0.0dev,!=3.20.0,!=3.20.1,!=4.21.0,!=4.21.1,!=4.21.2,!=4.21.3,!=4.21.4,!=4.21.5",

and

gapic-generator-python/gapic/templates/setup.py.j2

Lines 32 to 39 in 24a23a1

	dependencies = [
	"google-api-core[grpc] >= 1.34.1, <3.0.0dev,!=2.0.*,!=2.1.*,!=2.2.*,!=2.3.*,!=2.4.*,!=2.5.*,!=2.6.*,!=2.7.*,!=2.8.*,!=2.9.*,!=2.10.*",
	# Exclude incompatible versions of `google-auth`
	# See https://github.com/googleapis/google-cloud-python/issues/12364
	"google-auth >= 2.14.1, <3.0.0dev,!=2.24.0,!=2.25.0",
	"proto-plus >= 1.22.3, <2.0.0dev",
	{# Explicitly exclude protobuf versions mentioned in https://cloud.google.com/support/bulletins#GCP-2022-019 #}
	"protobuf>=3.19.5,<5.0.0dev,!=3.20.0,!=3.20.1,!=4.21.0,!=4.21.1,!=4.21.2,!=4.21.3,!=4.21.4,!=4.21.5",

As part of this upgrade, we should drop support for protobuf 3.x, and deprecate support for 4.x shortly after 5.x has been released in generated clients.

[vchudnov-g]

One customer issue that would benefit from this: googleapis/google-cloud-python#12560