Add since/private to substr polyfill

Author: dmsnell

src/wp-includes/compat-utf8.php

@@ -346,6 +346,9 @@ function _wp_utf8_codepoint_count( string $text, ?int $byte_offset = 0, ?int $ma
  * to the maximal subpart rule. This function is a fallback method
  * for calling `mb_strlen( $text, 'UTF-8' )`.
  *
+ * @since 6.9.0
+ * @access private
+ *
  * @param string $text              Count bytes of span in this text.
  * @param int    $at                Start counting at this byte offset.
  * @param int    $max_code_points   Stop counting after this many code points