feature: code cleanup after docdb removal

[itsmostafa]

Clean up database connection get_db() function

🗣 Description

Clean up code and update format on connection to the database Replaced all env vars with one env var MONGO_URI Also removed the now defunct env var MONGO_TYPE

The format to connect in your local environment will be: MONGO_URI=mongodb://root:really_secure_password123@mongodb:27017

💭 Motivation and context

DocumentDb is no longer used and instead connect to a database cluster in Mongo Atlas

🧪 Testing

tested connection locally

✅ Pre-approval checklist

• [x] This PR has an informative and human-readable title.
• [x] Changes are limited to a single goal - eschew scope creep!
• [x] I have read the CONTRIBUTING document.
• [x] These code changes follow cisagov code standards.
• [x] All relevant repo and/or project documentation has been updated to reflect the changes in this PR.
• [x] All new and existing tests pass.