[skip ci] Update UPGRADING.INTERNALS for #21607

Author: LamentXU123

UPGRADING.INTERNALS

@@ -145,6 +145,9 @@ PHP 8.6 INTERNALS UPGRADE NOTES
     (sendfile, splice, copy_file_range, TransmitFile), and is now used by
     php_stream_copy_to_stream_ex(). The mmap-based copy fallback was removed.
   . Added zend_string_equals_cstr_ci().
+  . Added zend_cstr_append_char(), zend_cstr_concat(), and
+    zend_cstr_concat3() as helper functions to allocate NUL-terminated raw C
+    strings from one or more buffers.
   . Added zend_string_ends_with() and related variants.
   . Added trait support for internal classes.