increase cabal-version to work with recent cabal

It started complaining about custom setup needing too old a version of
cabal, a very confusing error message.

1.12 is the version of Cabal on the i386ancient builder.

Sponsored-by: Jack Hill on Patreon

git-annex.cabal

 Name: git-annex
 Version: 10.20220624
-Cabal-Version: >= 1.10
+Cabal-Version: 1.12
 License: AGPL-3
 Maintainer: Joey Hess <id@joeyh.name>
 Author: Joey Hess