Add dependabot to manage maven dependencies

[shivarm]

see-https://docs.github.com/en/code-security/dependabot/dependabot-version-updates/configuration-options-for-the-dependabot.yml-file

[johnaohara]

we used to have dependabot enabled on this repo, but iirc @jesperpedersen removed it. Jesper, what was the reason for removing dependabot?

[jesperpedersen]

It is extremely hard to track binary signatures between dependencies, and dependabot has no concept of this.

Rejected.