Consider making `futureProofEnums` the default?

### Is your feature request related to a problem? Please describe.

In order to write [robust applications](https://github.com/graphql/graphql.github.io/pull/2434), it is important that application code handles unknown future values for enums.

### Describe the solution you'd like

Making `futureProofEnums` the default would force consumers to handle unknown cases, making them more robust to schema evolutions.

~~The current value is named `%future enum value` but I would recommend `__UNKNOWN` instead which feels a little bit more inline with the usual GraphQL conventions.~~

Edit: never mind, `__UNKNOWN` is not what we want. but maybe `%do not use this value, add a default case instead`?

Relay has [a linter rule to detect that all switches must also have a `default:` case](https://relay.dev/docs/getting-started/lint-rules/#relayno-future-added-value)