ci: automate merging releases to staging (resolves #2349)

[jobara]

Resolves #2349

• Will automerge a release to staging
• Provides a workflow that can be triggered to merge a release to production

Prerequisites

If this PR changes PHP code or dependencies:

• I've run composer format and fixed any code formatting issues.
• I've run composer analyze and addressed any static analysis issues.
• I've run php artisan test and ensured that all tests pass.
• I've run composer localize to update localization source files and committed any changes.

If this PR changes CSS or JavaScript code or dependencies:

• I've run npm run lint and fixed any linting issues.
• I've run npm run build and ensured that CSS and JavaScript assets can be compiled.