templates: removes `DATABASE_URI` env var from `with-vercel-website` template .env.example (#10098)

### What?

Previously, the `with-vercel-website` template included a `DATABASE_URI`
env var in the `.env.example` file - which was unneeded.

### Why?

The `with-vercel-website` template uses a `POSTGRES_URL` env var for the
db connection string env var instead.

### How?

Removes the `DATABASE_URI` env var from the .env.example file.

Also, updates the `DATABASE_URI` db string names in the following
templates from `payloadtests` to `your-database-name` for a more generic
/ clear name:
- with-postgres
- with-vercel-mongodb
- with-vercel-postgres
- with-vercel-website