Inaccurate JSON schema for template-parts endpoint

[johnbillion]

The schema provided by the wp/v2/template-parts REST API endpoint has some inaccuracies:

1. The origin property can be null as well as a string. This property maps directly to WP_Block_Template::origin which is string|null.
2. The modified property can be false as well as a date-time string.

Both of these properties can be seen with null/false values by performing an authenticated request to /wp/v2/template-parts on a freshly installed vanilla WordPress site.

Any updates will need unit test coverage.

Here's an untested update to the modified property to replace its type and format.

'oneOf' => array(
	array(
		'type'   => 'string',
		'format' => 'date-time'
	),
	array(
		'type' => 'boolean',
		'enum' => array( false ),
	)
),

Fix as suggested by John