Add dependabot config to check for updates for GHA workflows. (#10342)
This will automatically create PRs to update any GitHub Actions we are
using when new versions are published. It is set to check weekly,
defaults to limiting such PRs to 3 at a time, and will lable them
apprpriately with ‘no changelog’ and ‘area/ci’.
This is intended to reduce the manual overhead of maintaining our CI.
Austin S. Hemmelgarn