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!

[colinleach]

I'm unsure what to do with this and similar PRs. Merging is blocked, pending approval by maintainers-admin, but apparently that is still just Erik. Should this now be @exercism/cross-track-maintainers? If so, @IsaacG is part of that.

[colinleach]

It lets me add myself as a reviewer, but I can't remove Erik.

[BNAndras]

That seems odd. I’ve merged quite a few of these PRs as a cross-track-maintainer member. What if we make a new PR with the changes from this PR and then close this one?

[colinleach]

Sure, whatever works.

Unfortunately this may also be necessary for PRs #767, #772, #773, #775. I'll make new PRs.

[IsaacG]

I asked about that here (restricted thread): https://forum.exercism.org/t/maintained-vs-single-maintained-vs-unmaintained-tracks/12636/69

[colinleach]

Did you get a reply? (I don't have read access to this thread)

[IsaacG]

Did you get a reply? (I don't have read access to this thread)

Not yet. Jeremy and Erik are usually less active on the weekend.

[ErikSchierboom]

See #778