serve stale feature to keep the DNS resource records beyond TTL to return them as stale records in case of upstream server is not reachable or returns negative response.
SD_RESOLVED_NO_STALE flag has been added to disable serving stale records via dbus.
added serve stale test cases to TEST-75-RESOLVED
Fixes: #21815
The knot.service on Ubuntu Jammy loads an env file which we didn't
install, causing the service to fail:
```
knot.service: Will spawn child (service_enter_start_pre): /usr/sbin/knotc
knot.service: Failed to load environment files: No such file or directory
knot.service: Failed to run 'start-pre' task: No such file or directory
knot.service: Failed with result 'resources'.
knot.service: Service will not restart (restart setting)
```
