extend dependabot updates schedule interval from weekly to monthly

[slimreaper35]

issue: https://github.com/containerbuildsystem/cachi2/issues/450

Maintainers will complete the following section

• [ ] Commit messages are descriptive enough
• [ ] Code coverage from testing does not decrease and new code is covered
• [ ] Docs updated (if applicable)
• [ ] Docs links in the code are still valid (if docs were updated)

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• approve GitHub Actions workflows by clicking a button
• approve the Red Hat Trusted App Pipeline container build by commenting /ok-to-test (as is the standard for Pipelines as Code)