ci: Add support for GitHub's merge queue

[alessfg]

Proposed changes

This PR adds support for using GitHub's merge queue.

Checklist

Before creating a PR, run through this checklist and mark each as complete:

• I have read the contributing guidelines
• I have signed the F5 Contributor License Agreement (CLA)
• I have run the update.sh script and ensured all entrypoint/Dockerfile template changes have been applied to the relevant image entrypoint scripts & Dockerfiles
• I have tested that the NGINX Docker unprivileged image builds and runs correctly on all supported architectures on an unprivileged environment (check out the README for more details)
• I have updated any relevant documentation (README.md)

[Copilot]

Pull Request Overview

This PR adds support for GitHub's merge queue feature across all workflow files. The merge queue is a GitHub feature that helps ensure that changes are tested together before being merged to the main branch, improving the reliability of the CI/CD process.

• Adds merge_group: trigger to all GitHub Actions workflow files
• Enables workflows to run when pull requests are added to the merge queue
• Maintains existing triggers (pull_request, schedule, workflow_dispatch) while extending functionality

Reviewed Changes

Copilot reviewed 4 out of 4 changed files in this pull request and generated no comments.

File	Description
.github/workflows/debian-stable.yml	Adds merge_group trigger to enable workflow execution in GitHub's merge queue
.github/workflows/debian-mainline.yml	Adds merge_group trigger to enable workflow execution in GitHub's merge queue
.github/workflows/alpine-stable.yml	Adds merge_group trigger to enable workflow execution in GitHub's merge queue
.github/workflows/alpine-mainline.yml	Adds merge_group trigger to enable workflow execution in GitHub's merge queue

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