Add Dependabot configuration to check for dependencies updates

[EwoutH]

Adds a Dependabot configuration which checks daily if new Gradle or GitHub Actions dependencies are available. If so, it opens a PR with that Dependency update.

See Configuration options for dependency updates for more about the exact configuration.

[ehoogend]

Hi @EwoutH , thanks for the PR. I've discussed this in the team and we will not be using Dependabot for the time being for the following reasons:

• It doesn't fit our current development process. Our main repository is a private Azure DevOps repository. Having regular automated PRs to merge from Github would become too cumbersome/time-consuming. If we were to use Dependabot or similar tools it would be on the private repo.
• We want to manage the dependencies in a more conscious and pro-active way, so by planning and discussing during the sprint. The risk with automated regular PRs like this is that the human response over time often comes down to either "just ignore" or "always say yes".

In other news: we have worked on updating several dependencies in the current sprint. These changes should become visible on Github soon as well.

[EwoutH]

Thanks for the extensive explanation, sounds like a sensible solution.