FR(python): Upgrade the `default_python_version`

[parthea]

Python 3.8 is close to the end of life date in https://devguide.python.org/versions/#status-of-python-versions

We should bump the default_python_version to Python 3.12 or another version which is supported.

synthtool/synthtool/gcp/common.py

Lines 266 to 268 in 373d00f

	# Set default Python versions for noxfile.py
	if "default_python_version" not in kwargs:
	kwargs["default_python_version"] = "3.8"