Change: rebrand 1.12.0 to 12.0

One question that keeps popping up: "when do we release 2.0?".
NewGRF will force that at least 1.16 will be 2.0, but to not wait
for this, let's drop the "1." and be for ever done with that
conversation.

We are following in the footstep of giants here.

Doc: for over 10 years now, we do not load the exact AI version first (#8431)

See commit fae34ee7 for details. The documentation simply never
got updated.