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 unmaintained tracks, we are requiring it to be merged.

Thanks!

[Bajger]

@ErikSchierboom "For maintained track repos, maintainers can choose to close this PR unmerged." - does it make sense to this single-maintainer repo to keep interval to daily?

[ErikSchierboom]

does it make sense to this single-maintainer repo to keep interval to daily?

🤷 Depends on what you want. You'll need to with the PRs.

[Bajger]

Since cadence on this repo is quite sparse, I wish (if you agree) to keep it daily. To what I understood from Jeremy this repo (and test runner repo) will be kept in maintained-solitary mode and reviewed by cross-track maintainers?

[ErikSchierboom]

That is correct. It's fine to keep it at daily then