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.

cabal-testsuite/PackageTests/CustomWithoutCabal/cabal.out

 # cabal new-build
-Warning: <ROOT>/custom-setup-without-cabal.cabal: This package requires at least Cabal version 99999
 Resolving dependencies...
 In order, the following will be built:
  - custom-setup-without-cabal-1.0 (lib:custom-setup-without-cabal) (first run)

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

+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/cabal.out

 # cabal new-build
-Warning: <ROOT>/custom-setup/custom-setup.cabal: This package requires at least Cabal version 99999
 Resolving dependencies...
 In order, the following will be built:
  - Cabal-99999 (lib:Cabal) (first run)

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

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

cabal-testsuite/PackageTests/Regression/T3436/sandbox.out

@@ -10,7 +10,6 @@ Installing library in <PATH>
 Installed Cabal-99998
 # cabal sandbox add-source
 # cabal install
-Warning: custom-setup/custom-setup.cabal: This package requires at least Cabal version 99999
 Resolving dependencies...
 Configuring Cabal-99999...
 Preprocessing library for Cabal-99999..