feat: Make GUC names for headers, cookies and jwt claims compatible with PostgreSQL v14

Getting the value for a header GUC on PostgreSQL v14 is done using `current_setting('request.headers')::json->>'name-of-header'` and in a similar way for `request.cookies` and `request.jwt.claims`

PostgreSQL versions below 14 can opt in to the new JSON GUCs by setting the `db-use-legacy-gucs` config option to false (true by default)

src/PostgREST/Config/PgVersion.hs

@@ -12,6 +12,7 @@ module PostgREST.Config.PgVersion
   , pgVersion114
   , pgVersion121
   , pgVersion130
+  , pgVersion140
   ) where
 
 import qualified Data.Aeson as JSON
@@ -58,3 +59,6 @@ pgVersion121 = PgVersion 120001 "12.1"
 
 pgVersion130 :: PgVersion
 pgVersion130 = PgVersion 130000 "13.0"
+
+pgVersion140 :: PgVersion
+pgVersion140 = PgVersion 140000 "14.0"