Remove Legacy MySQL Code and Configuration

[aaronskiba]

Please correct me if I'm wrong, but it appears that all app instances based off of DMPRoadmap have transitioned to PostgreSQL. If so, then it should be in our best interest to remove all legacy MySQL code / configuration and fully transition to PostgreSQL. This work includes:

• Removing any MySQL gems
• Disabling the MySQL workflow (https://github.com/DMPRoadmap/roadmap/actions/workflows/mysql.yml) and corresponding file(s)
• Removing any MySQL-specific configurations (e.g. database.yml)
• Removing any code that is specific to MySQL queries or features
• Updating the documentation to remove any mentions of MySQL