logs: lower pnpm lock file error to info

lib/manager/npm/post-update/pnpm.js

@@ -73,7 +73,7 @@ async function generateLockFile(tmpDir, env) {
       'Generated lockfile'
     );
   } catch (err) /* istanbul ignore next */ {
-    logger.warn(
+    logger.info(
       {
         cmd,
         err,