style(pdk): rename some variables (#14063)

Follow up of #14025

KAG-6036

kong/pdk/vault.lua

@@ -1443,10 +1443,10 @@ local function new(self)
     local conf = self.configuration
 
     local not_dbless = conf.database ~= "off"  -- postgres
-    local dp_with_inc_sync = conf.role == "data_plane" and
+    local dp_with_rpc_sync = conf.role == "data_plane" and
                              conf.cluster_rpc_sync
 
-    return not_dbless or dp_with_inc_sync
+    return not_dbless or dp_with_rpc_sync
   end
 
   local initialized