fix: reduce Renovate prConcurrentLimit to 5

PR .github#32 proposed adding a new renovate.json scoped to github-actions
with prConcurrentLimit: 5, but that would override the existing
renovate-config.json and silently drop npm dependency updates.

Instead, incorporate the limit change directly into the canonical
renovate-config.json which already covers both npm and github-actions.

Co-authored-by: Gandalf the Greybeard <gandalf@privilegedescalation.ai>
Co-authored-by: Paperclip <noreply@paperclip.ing>

renovate-config.json

@@ -3,7 +3,7 @@
   "extends": ["config:recommended"],
   "baseBranches": ["main"],
   "schedule": ["every weekend"],
-  "prConcurrentLimit": 10,
+  "prConcurrentLimit": 5,
   "packageRules": [
     {
       "matchManagers": ["npm"],