fix: Allow protobuf 6.x

[parthea]

See https://pypi.org/project/protobuf/6.30.0/
See googleapis/gapic-generator-python#2347 for removing dev in setup.py

Adding do not merge as it's not clear if protobuf 6.x is being tested in the prerelease_deps nox session.

python-bigquery/noxfile.py

Lines 358 to 364 in f8572dd

	@nox.session(python=SYSTEM_TEST_PYTHON_VERSIONS)
	@_calculate_duration
	def prerelease_deps(session):
	"""Run all tests with prerelease versions of dependencies installed.
	https://github.com/googleapis/python-bigquery/issues/95
	"""

[parthea]

This still requires more work to ensure that the latest pre-release versions are installed.