Fix cabal-doctest version

ci/config.cabal.project

@@ -6,6 +6,7 @@ index-state:
     hackage.haskell.org 2023-10-16T12:18:14Z,
     head.hackage HEAD
 
+constraints: cabal-doctest >= 1.0.9
 constraints: th-abstraction >= 0.4
 
 -- aeson is now quite old, don't attempt to use it