Bump minimum virtualenv to 20.14.1

[johnthagen]

virtualenv dependencies, for example

• Update for platformdirs v4 pypa/virtualenv#2666

can cause dependency resolution of virtualenv to drop to an older version (for example 20.4.7) that can cause nox to fail due to using deprecated setuptools behavior. For example:

• https://github.com/johnthagen/python-blueprint/actions/runs/6834646086/job/18587635299

Bump nox's minimum virtualenv version to avoid dependency resolution selecting a very outdated virtualenv, which isn't tested and can fail.

virtualenv 20.14.1 was released 2022-04-11.

[henryiii]

There are also some workaround in the codebase that could be dropped by increasing the min virtualenv version. Like

nox/nox/virtualenv.py

Lines 353 to 355 in 5c82dc5

	if not os.path.isfile(path):
	# virtualenv < 20.0 does not create pyvenv.cfg
	old_env = "virtualenv"