ci/renovate: limit gomod digest updates to once every two weeks

[roobre]

Go modules that are not versioned create PRs continuously, e.g.:

• https://github.com/grafana/synthetic-monitoring-agent/pull/781
• https://github.com/grafana/synthetic-monitoring-agent/pull/778

This PR throttles digest-type updates to go modules so they only happen one out of two Mondays.