Add `removeSourceBranch` parameter to `git-open-pr` step

[SykeElias]

Description:
When using git-open-pr with generateTargetBranch: true, many temporary branches are created.

Currently, GitLab's project-level setting "Delete source branch by default" doesn't apply when PRs are created via API.

Proposed Solution:
Add a removeSourceBranch parameter to git-open-pr:

- uses: git-open-pr
  config:
    sourceBranch: ${{ outputs.push.branch }}
    targetBranch: main
    removeSourceBranch: true  # ← NEW parameter

This would pass remove_source_branch: true to the GitLab/GitHub API when creating the MR/PR.

Use Case:
Avoiding branch explosion when using generateTargetBranch: true for promotion workflows.

API Support:

• GitLab API: https://docs.gitlab.com/ee/api/merge_requests.html#create-mr (remove_source_branch)
• GitHub API: Doesn't directly support this, but can use auto_merge workaround

[Brightside56]

related to #5068