Remove deprecated `config.db.useMigrations`, change keystone prisma behaviour

[dcousens]

This pull request removes config.db.useMigrations, which was deprecated in https://github.com/keystonejs/keystone/pull/8046, but perhaps a better description of why is needed to help with updating your workflow.

I'll add a draft of that workflow here into our documentation soon. The short of it could probably be summarised in the following text (work in progress):

# development lifecycle
$ keystone dev
  1. Keystone updates your schemas (GraphQL, Prisma, Typescript)
  2. Prisma pushes any updates to your database (unless you use --no-db-push)
  3. Go to 1, until ready

$ keystone prisma migrate dev --create-only
  4. Generate any database migrations, editing as required

$ keystone start --with-migrations
  5. Test your migrations
  6. If happy, continue, otherwise go back, using `keystone prisma migrate reset` as needed

# ...

# deploying
$ keystone build

$ keystone start --with-migrations

Any feedback is appreciated, as we move to integrate this workflow into the documentation.

References

• https://www.prisma.io/docs/orm/prisma-migrate/workflows/development-and-production
• https://www.prisma.io/docs/orm/reference/prisma-cli-reference#options-6