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/App.hs

@@ -198,7 +198,7 @@ postgrestResponse conf maybeDbStructure jsonDbS pgVer pool time req = do
 
   runDbHandler pool (txMode apiRequest) jwtClaims (configDbPreparedStatements conf) .
     Middleware.optionalRollback conf apiRequest $
-      Middleware.runPgLocals conf jwtClaims handleReq apiRequest jsonDbS
+      Middleware.runPgLocals conf jwtClaims handleReq apiRequest jsonDbS pgVer
 
 runDbHandler :: SQL.Pool -> SQL.Mode -> Auth.JWTClaims -> Bool -> DbHandler a -> Handler IO a
 runDbHandler pool mode jwtClaims prepared handler = do