ci: constrain megaparsec < 9.8 across all GHCs

Broaden the previous GHC < 9.10 constraint to the whole matrix.
megaparsec-9.8.0 breaks the build two different ways:

  * GHC < 9.10: fails to compile ("Variable not in scope: foldl'"),
    relying on foldl' being re-exported from Prelude (base-4.20 / GHC 9.10).
  * GHC >= 9.10: compiles, but changes parse-error rendering (the `^` caret
    under the offending token), breaking the expected strings in
    Database.Persist.Quasi's test suite (3 failures in QuasiSpec).

The earlier conditional only fixed the compile error; pin unconditionally
so every job uses the pre-9.8 error format the tests expect.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

Author: jkeuhlen

cabal.project

@@ -20,11 +20,15 @@ allow-newer:
     , bytestring-lexing:base
     , postgresql-simple-interval:persistent
 
--- megaparsec 9.8.0 uses an unqualified `foldl'`, which is only re-exported from
--- Prelude as of base-4.20 (GHC 9.10). On older GHCs it fails to compile with
--- "Variable not in scope: foldl'". Pin to < 9.8 there; newer GHCs build it fine.
-if impl(ghc < 9.10)
-    constraints: megaparsec < 9.8
+-- megaparsec 9.8.0 breaks the build in two ways:
+--   * On GHC < 9.10 it fails to compile ("Variable not in scope: foldl'"),
+--     because it relies on `foldl'` being re-exported from Prelude, which only
+--     happens as of base-4.20 (GHC 9.10).
+--   * On GHC >= 9.10 it compiles but changes how parse errors are rendered
+--     (the `^` caret under the offending token), which breaks the expected
+--     strings in Database.Persist.Quasi's test suite.
+-- Pin to < 9.8 across the whole matrix until the suite is updated upstream.
+constraints: megaparsec < 9.8
 
 source-repository-package
     type: git