refactor: fix app.js and update server.js

[matthewsong18]

useNewUrlParser and useUnifiedTopology is depreciated. It should look like the following code instead. However, this code should be inside server.js instead.

mongoose.connect("mongodb://localhost:27017/db")

_Originally posted by @matthewsong18 in https://github.com/matthewsong18/csc337-final-project/pull/41#discussion_r1870340848_

• [ ] Fix app.js
• [ ] Update server.js

[matthewsong18]

https://github.com/matthewsong18/csc337-final-project/pull/41#discussion_r1870341447