build: add Dependabot auto-merge workflow

[milkshakeuk]

This pull request replaces the previous Dependabot auto-approve workflow with a new, more robust auto-merge workflow. The new workflow ensures that Dependabot pull requests for minor and patch updates are automatically approved and merged only after all required checks have passed and the PR is up-to-date with the base branch. It also handles concurrency issues and provides detailed status comments on the PR.

Replacement and enhancement of Dependabot automation:

• Removed the old .github/workflows/dependabot-auto-approve.yml workflow, which only auto-approved eligible Dependabot PRs.
• Added a new .github/workflows/dependabot-auto-merge.yml workflow that not only auto-approves but also fast-forward merges eligible Dependabot PRs once all checks are complete and the PR is strictly up-to-date.

Key features of the new workflow:

• Uses a GitHub App token for secure permissions and to interact with protected branches and checks.
• Waits for all required checks (except itself) to complete and verifies that all have succeeded before proceeding to merge.
• Automatically approves PRs if not already approved and labels them as "auto-approved".
• Handles merge conflicts and out-of-date branches by requesting Dependabot to recreate the PR if necessary.
• Posts a detailed comment on the PR summarizing the success or failure of the auto-merge process, updating the comment if it already exists.

[github-actions]

🎉 Success! 🎉

PR successfully Fast Forward merged in ✅.