reenable and adopt haskell-src

I am now maintainer of haskell-src and released revision 1.0.3.1-r2
that accepts happy-1.20.

build-constraints.yaml

@@ -28,6 +28,7 @@ packages:
         - Agda < 0  # https://github.com/commercialhaskell/stackage/issues/5826
         - agda2lagda
         - ListLike
+        - haskell-src
 
     "Diogo Biazus <diogo@biazus.ca>":
         - hasql-notifications
@@ -4660,7 +4661,6 @@ packages:
         - haskell-gi-overloading
         - haskell-lexer
         - haskell-lsp-types
-        - haskell-src < 0 # ghc 8.10
         - haskell-src-exts
         - haxl < 0 # via time-1.9.3
         - heap