Add steps to pull requests created by Prepare for minor release workflow

[bberndt-uaz]

Motivation

The Prepare for minor release workflow creates two pull requests. Currently, both of these need additional manual changes before they can be merged as part of the process for creating the alpha1 release for a new Quickstart minor version.

Proposed Resolution

Describe the solution you'd like

For the "Prepare main branch for new minor release branch x.y.z" pull request:

• Update .github/workflows/ci.yml to add the new minor release branch. (Removal of any unsupported release branches will remain a manual process.)

For the "Prepare repository for new minor release branch x.y.z" pull request:

• Update .github/workflows/ci.yml to add the new minor release branch. (Removal of any unsupported release branches will remain a manual process.)
• Change SCAFFOLD_BRANCH in .tugboat/config.yml from main to the new minor release branch.