chore(renovate): add pinDigests to template repo renovate.json

Ensures repos created from this template start with GitHub Actions SHA pinning
configured. Matches the pinDigests: true change being applied to all existing
plugin repos (PRI-757).

Without this, new repos created from the template would need a follow-up PR
to add pinDigests.

renovate.json

@@ -4,6 +4,7 @@
   "baseBranches": ["main"],
   "schedule": ["every weekend"],
   "prConcurrentLimit": 10,
+  "pinDigests": true,
   "packageRules": [
     {
       "matchManagers": ["npm"],
@@ -17,3 +18,4 @@
     }
   ]
 }
+