scripts: sphinx-pre-install: change the warning for version < 2.4.4

As requested by Jon, change the version check, in order to not
emit a warning if version is >= 1.7.9, but below 2.4.4.

After this patch, if someone used an older version, it will
say:

	./scripts/sphinx-pre-install
	Sphinx version 1.7.9
	Note: It is recommended at least Sphinx version 2.4.4 if you need PDF support.
	Detected OS: Fedora release 31 (Thirty One).

	To upgrade Sphinx, use:

		/devel/v4l/docs/sphinx_1.7.9/bin/python3 -m venv sphinx_2.4.4
		. sphinx_2.4.4/bin/activate
		pip install -r ./Documentation/sphinx/requirements.txt

	If you want to exit the virtualenv, you can use:
		deactivate

	All optional dependencies are met.
	Needed package dependencies are met.

If Sphinx is not detected at all, it

Signed-off-by: Mauro Carvalho Chehab <mchehab+huawei@kernel.org>
Link: https://lore.kernel.org/r/79584d317ba16f5d4f37801c5ee57cf04085f962.1587478901.git.mchehab+huawei@kernel.org


Signed-off-by: Jonathan Corbet <corbet@lwn.net>