Enable dependabot and scala-steward

[tuxji]

This can wait until after the release of daffodil-sbt 1.0.0.

How to enable dependabot:

• Copy dependabot.yml from daffodil/.github to daffodil-sbt/.github, remove "docker", commit, and open a pull request. Once that PR is merged, @dependabot will check periodically for version updates.

How to enable scala-steward:

• Open a pull request that adds apache/daffodil-sbt to repos-github.md (edit). Once that PR is merged, @scala-steward will check periodically for version updates.

[Optional] How to enable code scanning:

• Configure one of the 74 code scanning workflows offered by GitHub.

[stevedlawrence]

Scala-steward is now enabled: VirtusLab/scala-steward-repos#400