chore: updates `.env.example` env vars along side `.env` vars based on selected DB in cpa

[PatrikKozak]

What?

Previously, the create-payload-app install would properly update the env vars in the new created .env file but kept outdated env vars in the .env.example file.

I.e

If selecting a postgres DB for the blank or website template:

.env --> DATABASE_URI --> postgres://postgres:<password>@127.0.0.1:5432/test-cpa

.env.example --> DATABASE_URI --> mongodb://127.0.0.1/payload-template-blank-3-0

Now

.env --> DATABASE_URI --> postgres://postgres:<password>@127.0.0.1:5432/test-cpa

.env.example --> DATABASE_URI --> postgres://postgres:<password>@127.0.0.1:5432/test-cpa