Change Request: Remove `eslintrc` support

As of the upcoming ESLint v10, it appears we will drop support for the `eslintrc` mode from core.

As part of that process, I'm proposing to remove `eslintrc` support from the `create-config` package.

There appears to be `eslintrc`-related logic throughout the `create-config` package:

https://github.com/eslint/create-config/blob/61524d16d23998b8714b2e3bdcf5ea45da09a57e/bin/create-config.js#L48

https://github.com/eslint/create-config/blob/61524d16d23998b8714b2e3bdcf5ea45da09a57e/lib/config-generator.js#L55

https://github.com/eslint/create-config/blob/61524d16d23998b8714b2e3bdcf5ea45da09a57e/lib/config-generator.js#L294-L295

https://github.com/eslint/create-config/blob/61524d16d23998b8714b2e3bdcf5ea45da09a57e/lib/config-generator.js#L319-L325

https://github.com/eslint/create-config/blob/61524d16d23998b8714b2e3bdcf5ea45da09a57e/lib/config-generator.js#L338