Don't use cabal-version: >= 99999 in test-cases

Since we now reject unknown spec-versions in the solver, these test-cases
were broken; this replaces the 2 occurences by `cabal-version: 2.0` instead.

(cherry picked from commit 45c102d)

Author: hvr

cabal-testsuite/PackageTests/CustomWithoutCabal/custom-setup-without-cabal.cabal

@@ -1,7 +1,7 @@
+cabal-version: 2.0
 name: custom-setup-without-cabal
 version: 1.0
 build-type: Custom
-cabal-version: >= 99999
 
 custom-setup
   setup-depends: base

cabal-testsuite/PackageTests/Regression/T3436/custom-setup/custom-setup.cabal

@@ -1,7 +1,7 @@
+cabal-version: 2.0
 name: custom-setup
 version: 1.0
 build-type: Custom
-cabal-version: >= 99999
 
 custom-setup
   setup-depends: base, Cabal >= 99999