Squash and Merge Pull Request when Promoting in DevOps Center

We should be able to set which Merge Type we want to use in the Settings or Pipeline, so we guarantee consistent behaviour between DevOps Center and GitHub.

Squash and Merge makes the git history much cleaner and allows us to keep the branch protection rule "Require linear history"

Review the DevOps Center promotion flow, its Settings or Pipeline configuration, and the GitHub merge behavior. No files or tests are named, so first locate where promotion merge types are selected. Done means users can choose a merge type and promotions consistently use it, including support for Squash and Merge with linear-history protection.

Written by the indexing model from the issue text.