Update theme_has_theme_json to reflect WP 6.3 changes

[westonruter]

1. Update theme_has_theme_json method to reflect what was actually committed.
2. Uncomment the call to wp_theme_has_theme_json() if defined.

amp-wp/src/ReaderThemeSupportFeatures.php

Lines 581 to 626 in d7a6af8

	private function theme_has_theme_json() {
	// @TODO: Uncomment this once `wp_theme_has_theme_json()` caching is fixed.
	// if ( function_exists( 'wp_theme_has_theme_json' ) ) {
	// return wp_theme_has_theme_json();
	// }
	static $theme_has_support = null;
	static $prev_stylesheet_directory = null;
	static $prev_template_directory = null;
	$stylesheet_directory = get_stylesheet_directory();
	$template_directory = get_template_directory();
	// Make sure that the cached $theme_has_support value is reset when the theme changes.
	if ( null !== $theme_has_support && (
	$stylesheet_directory !== $prev_stylesheet_directory ||
	$template_directory !== $prev_template_directory
	) ) {
	$theme_has_support = null;
	}
	$prev_stylesheet_directory = $stylesheet_directory;
	$prev_template_directory = $template_directory;
	if (
	null !== $theme_has_support &&
	// Ignore static cache when the development mode is set to 'theme', to avoid interfering with
	// the theme developer's workflow.
	( function_exists( 'wp_get_development_mode' ) && wp_get_development_mode() !== 'theme' )
	) {
	return $theme_has_support;
	}
	// This is the same as get_theme_file_path(), which isn't available in load-styles.php context.
	if ( $stylesheet_directory !== $template_directory && file_exists( $stylesheet_directory . '/theme.json' ) ) {
	$path = $stylesheet_directory . '/theme.json';
	} else {
	$path = $template_directory . '/theme.json';
	}
	/** This filter is documented in wp-includes/link-template.php */
	$path = apply_filters( 'theme_file_path', $path, 'theme.json' );
	$theme_has_support = file_exists( $path );
	return $theme_has_support;
	}

[pooja-muchandikar]

QA Passed ✅

Checked the issue, and verified it on WP6.1 and WP6.2 and WP6.3 beta versions and the 2023 theme with the legacy mode appears to be working fine.