docs: Deprecate the pseries-2.12 machines

pSeries machines before 3.0 have complex migration back
compatibility code we'd like to get ride of. The last
one is 2.12, which is 6 years old. We just deprecated up
to the 2.11 machine in commit 1392617d35 ("spapr: Tag
pseries-2.1 - 2.11 machines as deprecated").
Take to opportunity to also deprecate the 2.12 machines.

Acked-by: Nicholas Piggin <npiggin@gmail.com>
Signed-off-by: Philippe Mathieu-Daudé <philmd@linaro.org>
Signed-off-by: Nicholas Piggin <npiggin@gmail.com>

docs/about/deprecated.rst

@@ -237,13 +237,13 @@ The Nios II architecture is orphan.
 The machine is no longer in existence and has been long unmaintained
 in QEMU. This also holds for the TC51828 16MiB flash that it uses.
 
-``pseries-2.1`` up to ``pseries-2.11`` (since 9.0)
+``pseries-2.1`` up to ``pseries-2.12`` (since 9.0)
 ''''''''''''''''''''''''''''''''''''''''''''''''''
 
-Older pseries machines before version 2.12 have undergone many changes
+Older pseries machines before version 3.0 have undergone many changes
 to correct issues, mostly regarding migration compatibility. These are
 no longer maintained and removing them will make the code easier to
-read and maintain. Use versions 2.12 and above as a replacement.
+read and maintain. Use versions 3.0 and above as a replacement.
 
 Arm machines ``akita``, ``borzoi``, ``cheetah``, ``connex``, ``mainstone``, ``n800``, ``n810``, ``spitz``, ``terrier``, ``tosa``, ``verdex``, ``z2`` (since 9.0)
 ''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''