Update dependabot updates interval to monthly

[meili-bot]

This PR is auto-generated.

Change dependabot configuration to be monthly. Make sure you got the right configuration after this automation, check the open-pull-requests-limit and the allow list of dependencies.

Related to: https://github.com/meilisearch/integration-guides/issues/283

[meili-bors[bot]]

Build succeeded:

• integration-tests (dart 3.0.0)
• linter-check
• pana