Change dependabot to a monthly cadence

[jeffcharles]

Description of the change

Changing Dependabot's cadence for all ecosystems to monthly.

Why am I making this change?

We don't need to update our dependencies more frequently unless there's a security concern.

Checklist

• I've updated the relevant CHANGELOG files if necessary. Changes to javy-cli and javy-core do not require updating CHANGELOG files.
• I've updated the relevant crate versions if necessary. Versioning policy for library crates
• I've updated documentation including crate documentation if necessary.