Refactor sync workflow to use Github App

[pdabelf5]

Proposed changes

This pull request updates the GitHub Actions workflows to improve security and modernize authentication for repository automation. The main changes involve removing the legacy Dependabot auto-merge workflow and refactoring the sync.yml workflow to use GitHub App tokens fetched from a vault, instead of personal access tokens.

Workflow deprecation:

• Removed the .github/workflows/dependabot-auto-merge.yml workflow, discontinuing automatic merging of Dependabot PRs.

Authentication and security improvements:

• Updated the sync.yml workflow to fetch GitHub App credentials from a vault using the get-from-vault action, and mint short-lived GitHub App tokens for each organization and repository, replacing the use of static personal access tokens.
• Added explicit id-token: write and contents: read permissions for jobs, following GitHub Actions security best practices.

Repository sync and label management updates:

• Refactored the label sync step to use the new GitHub App tokens and updated repository references from nginxinc to nginx where appropriate.
• Split repository sync steps to use the appropriate minted token for each organization, ensuring least-privilege access.

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).
• If applicable, I have added tests that prove my fix is effective or that my feature works.
• If applicable, I have checked that any relevant tests pass after adding my changes.
• I have updated any relevant documentation (README.md and/or CHANGELOG.md).