chore(renovate): allow 20 concurrent PR's

The config:base preset sets it to 10. That's a tad low when one
intentionally keeps bumps open (to reduce churn), for example until
they break something or a release is imminent.

renovate.json

@@ -1,6 +1,7 @@
 {
   "extends": [
     "config:base",
+    ":prConcurrentLimit20",
     ":preserveSemverRanges"
   ],
   "ignorePresets": [