Add RAT to dependabot config

[alamb]

Which issue does this PR close?

re #1587

Rationale for this change

while trying to create a release candidate this file was flagged as "not RAT":

(arrow_dev) alamb@MacBook-Pro-2:~/Software/arrow-datafusion$ GH_TOKEN=$ARROW_GITHUB_API_TOKEN ./dev/release/create-tarball.sh 7.0.0 1
Attempting to create  from tag 7.0.0-rc1
Draft email for dev@arrow.apache.org mailing list
....
Running rat license checker on /Users/alamb/Software/arrow-datafusion/dev/dist/apache-arrow-datafusion-7.0.0-rc1/apache-arrow-datafusion-7.0.0.tar.gz
NOT APPROVED: .github/dependabot.yml (apache-arrow-datafusion-7.0.0/.github/dependabot.yml): false
       1 unapproved licences. Check rat report: rat.txt

What changes are included in this PR?

Add apache license to a config file

Are there any user-facing changes?

no