docs/about: Mark the old pc-i440fx-2.0 - 2.3 machine types as deprecated

docs/about: Mark the old pc-i440fx-2.0 - 2.3 machine types as deprecated

As we've seen in the past, it's useful for deprecating old machine
types to finally be able to get of legacy code or do other clean-ups
(see e.g. commit ea985d235b868047 that was used to drop the PCI code in
the 128k bios binaries to free some precious space in those binaries).

So let's continue deprecating the oldest pc machine types. QEMU 2.3
has been released 8 years ago, so that's plenty of time since such
machine types have been used by default, thus deprecating pc-i440fx-2.0
up to pc-i440fx-2.3 should be fine nowadays.

Message-ID: <20231006075247.403364-1-thuth@redhat.com>
Reviewed-by: Philippe Mathieu-Daudé <philmd@linaro.org>
Signed-off-by: Thomas Huth <thuth@redhat.com>