Adjust `deploy-to-rails` script to add "bot-authored" label

What does this MR do?

Adds automation:bot-authored label to created MR.

What are the relevant issue numbers?

gitlab-org/gitlab#372398 (closed)

Checklist

• Ensure changes can be built upon without requiring a breaking change, see Building for Extensibility.
• Review and add/update tests for this feature/bug.
• Add an entry to the CHANGELOG if required, with the appropriate version. See Classifying Changes.
• Assign the MR to the appropriate person/people for review.