Remove the 'cpp' implementation once support for Protobuf 3.x is dropped.

[parthea]

See the code below which should be removed once support for Protobuf 3.x is dropped.

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

Lines 226 to 227 in 9310dd9

	if protobuf_implementation == "cpp" and session.python in ("3.11", "3.12", "3.13"):
	session.skip("cpp implementation is not supported in python 3.11+")

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

Lines 234 to 238 in 9310dd9

	# TODO(https://github.com/googleapis/synthtool/issues/1976):
	# Remove the 'cpp' implementation once support for Protobuf 3.x is dropped.
	# The 'cpp' implementation requires Protobuf<4.
	if protobuf_implementation == "cpp":
	session.install("protobuf<4")