.github: Enable Dependabot on this fork

Dependabot is configured at https://github.com/ArduPilot/mavlink/blob/master/.github/dependabot.yml, but a repo maintainer must enable it on this fork of the upstream https://github.com/mavlink/mavlink to keep our GitHub Actions up to date.
* https://github.blog/changelog/2022-11-07-dependabot-pull-requests-off-by-default-for-forks