DX: Rename `ComponentInputsForm` to `ComponentInstanceForm`

Overview

Ran into this (again) at #3523496: Block component instance form values not processed by validation/submit handlers. #3499703: Make all XB HTTP API routes consistently prefixed, ensure they all have OpenAPI specs, and tests to keep it so made the DX a lot clearer, but this was a bit that was out of scope there (it was focused on routes, nothing else).

Quoting XB's routing definitions:

# Forms: component instance and content entity.
# @see \Drupal\experience_builder\Theme\XBThemeNegotiator::applies()
experience_builder.api.form.component_inputs:
  path: '/xb/api/v0/form/component-instance/{entity_type}/{entity}'
  defaults:
    _form: 'Drupal\experience_builder\Form\ComponentInputsForm'
  …

This is inconsistent. And all docs refer to "component instance"s. So let's make all that consistent :)

Proposed resolution

Let's make it consistent:

# Forms: component instance and content entity.
# @see \Drupal\experience_builder\Theme\XBThemeNegotiator::applies()
experience_builder.api.form.component_instance:
  path: '/xb/api/v0/form/component-instance/{entity_type}/{entity}'
  defaults:
    _form: 'Drupal\experience_builder\Form\ComponentInstanceForm'
  …

User interface changes