Enable lockfile maintaince (#450)

Same as google/osv.dev#1505

Confirmed that it works as expected in osv.dev repository.

> Dependabot often has updates that don't show up in renovate bot.
That's because renovatebot doesn't directly update any transient
dependencies.
> 
> Enabling this option instructs renovatebot to relock the lockfiles,
which will bump all transient dependencies. See
https://docs.renovatebot.com/configuration-options/#lockfilemaintenance
and renovatebot/renovate#15762

renovate.json

@@ -8,6 +8,7 @@
   "labels": ["dependencies"],
   "postUpdateOptions": ["gomodTidy"],
   "osvVulnerabilityAlerts": true,
+  "lockFileMaintenance": { "enabled": true },
   "packageRules": [
     {
       "matchUpdateTypes": ["major"],