Cosinder adding dependabot for automatic dependency updates

[StefMa]

It's just a simple config file 🙃.
However I'm not sure if you want to deal with automatic dependency updates at this point.

[holzensp]

We're waiting until we're fully on GraalVM23 (for 0.26.0-dev we're now dropping JDK11 support) and Gradle 8.x, but then, yes, definitely want dependabot

[odenix]

I prefer renovate-bot (used by JUnit etc.), but either bot is a big step forward. Sending manual PRs with dependency updates is time-consuming for authors and reviewers, and I'd appreciate if it was automated away as much as possible. It only takes a few minutes!

[StefMa]

Dependabot can handle lock files now 🎉
https://github.blog/changelog/2025-06-24-dependabot-support-for-gradle-lockfiles-is-now-generally-available/