[Docs]: Replace Appsmith DB url from `APPSMITH_MONGODB_URI` to `APPSMITH_DB_URL` in env references

### Problem Statement

We are in a transition to move DB from `MongoDB` to `Postgres` that Appsmith uses to persist the data for all entities like users, workspaces, applications, datasources etc. We are making sure we have a fallback for our existing users to support the backward compatibility, but for any new installation we want to start consuming the updated env variable.

### Use Cases / User query reference

As of today we refer the DB url with env `APPSMITH_MONGODB_URI` which needs to be updated to `APPSMITH_DB_URL`.

### Audio/Video Recording

_No response_

### Product Requirements Document

_No response_

### Other Relevant Links

https://www.notion.so/appsmith/Move-to-Postgres-110125e2c94145848b67abd40b7a4a8c

### Checklist

- [X] Reviewed existing documentation to ensure this request is not duplicated.
- [X] Attached relevant media or supporting materials like links to the PRD, feature recordings, Slack or Discord links, and any other relevant resources.