Bump cabal-version to 2.2 (#6377)

In b3cfdc2c7 the license was changed to GPL-2.0-or-later which is an
SPDX expression, however cabal only interprets the license field as an
SPDX expression if cabal-version is 2.2 or later.

Starting with 2.2 cabal-version also has to be the first statement in
the .cabal file.

pandoc.cabal

@@ -1,6 +1,6 @@
+cabal-version:   2.2
 name:            pandoc
 version:         2.9.2.1
-cabal-version:   2.0
 build-type:      Simple
 license:         GPL-2.0-or-later
 license-file:    COPYING.md