[>=11.1.8] Remove `type: field.value.language` work-around once

Overview

In HEAD:

# @todo Remove when https://drupal.org/i/3516520 is fixed.
field.value.language:
  type: mapping
  label: 'Default value'
  mapping:
    value:
      type: string
      label: 'Value'

(Added in 📌 Get Options as buttons in Page Data form working Active .)

Good news: 🐛 LanguageItem lacks a schema for its default values Active landed on May 13, 2025!

Proposed resolution

1. Remove that from XB's config schema
2. Bump XB's core requirement to Drupal 11.1.8, or whichever the first release that contains the fix ends up being.

User interface changes

None.