ci(dependabot): change to monthly interval

[ErikSchierboom]

This PR changes the interval for dependabot updates to monthly.

Right now, the interval is usually set to daily, which results in (almost) daily notifications for Exercism maintainers, and frequent Docker updates to all tooling repos.

To reduce the maintenance burden, we are lowering the frequency to monthly. Due to Exercism's setup, it is unlikely that dependabot issues in our repositories need to be handled with any urgency, so we consider this to be a wise tradeoff.

For maintained track repos, maintainers can choose to close this PR unmerged. But for tooling repos and maintained tracks, we are requiring it to be merged.

Thanks!

[kotp]

For maintained track repos, maintainers can choose to close this PR unmerged. But for tooling repos and maintained tracks, we are requiring it to be merged.

This is directly contradictive, as for maintained track repos we have an option, but in the next sentence this group specifically does not have an option as it is required.

@ErikSchierboom I am merging, but this wording warrant some clarification or correction.