ci: constrain megaparsec < 9.8 on GHC < 9.10

CI builds recently started failing on GHC 8.8–9.8 with:

    Text/Megaparsec/Stream.hs:517:26: error:
        Variable not in scope: foldl'
    Failed to build megaparsec-9.8.0

megaparsec-9.8.0 uses an unqualified `foldl'`, which is only re-exported
from Prelude as of base-4.20 (GHC 9.10). Because CI runs `cabal v2-freeze`
fresh on every run (no committed freeze file), it floats to the newest
Hackage release, and 9.8.0 fails to compile on every GHC below 9.10.

Pin megaparsec < 9.8 for those compilers via a project conditional; GHC
9.10+ continue to use 9.8.0, which builds there without issue.

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

Author: jkeuhlen

cabal.project

@@ -20,6 +20,12 @@ 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
+
 source-repository-package
     type: git
     location: https://github.com/parsonsmatt/mysql