Background sync interval never adapts to user activity

[ChristophWurst]

Steps to reproduce

1. Add an account
2. Don't use it for more than three days
3. Check the cron job schedule

Expected behavior

Account sync job runs every 6h

Actual behavior

Account sync job runs every hour

Mail app version

5.11/5.12

Nextcloud version

No response

Mailserver or service

No response

Operating system

No response

PHP engine version

None

Nextcloud memory caching

No response

Web server

None

Database

None

Additional info

Feature was introduced with #11149 but never worked. Checked app and server code for it. Setting setInterval in run has no effect. Only the value set in the constructor does.