Fix #10042 Don't recommend deprecated/removed 'extensions:' field (#10078)

(cherry picked from commit dc9a7a02)

Co-authored-by: Mike Pilgrem <mpilgrem@users.noreply.github.com>

Cabal/src/Distribution/PackageDescription/Check/Target.hs

@@ -858,14 +858,14 @@ checkGHCOptions title t opts = do
       let ghcNoRts = rmRtsOpts opts
       checkAlternatives
         title
-        "extensions"
+        "default-extensions"
         [ (flag, prettyShow extension)
         | flag <- ghcNoRts
         , Just extension <- [ghcExtension flag]
         ]
       checkAlternatives
         title
-        "extensions"
+        "default-extensions"
         [ (flag, extension)
         | flag@('-' : 'X' : extension) <- ghcNoRts
         ]

changelog.d/issue-10042

+synopsis: Don't recommend deprecated/removed 'extensions:' field
+packages: Cabal
+prs: #10044
+issues: #10042
+
+description: {
+  When applicable, field 'default-extensions:' is recommended (rather than
+  deprecated/removed 'extensions:').
+}