Fix @since tag in docblock in WP_Theme_JSON_Data_Gutenberg. (#62425)

The `get_theme_json` method was actually added in v18.5.0, not  18.3.0. This corrects the docblock reference.

Co-authored-by: joemcgill <joemcgill@git.wordpress.org>
Co-authored-by: fabiankaegy <fabiankaegy@git.wordpress.org>

lib/class-wp-theme-json-data-gutenberg.php

@@ -72,7 +72,7 @@ public function get_data() {
 	/**
 	 * Return theme JSON object.
 	 *
-	 * @since 18.3.0
+	 * @since 18.5.0
 	 *
 	 * @return WP_Theme_JSON
 	 */