bin/gen_release_notes.py: fix conditional of bugfix

Previously this would result in the .0 warning be generated for .z > 0
and the .z == 0 would get the other message.

Fixes: 86079447
       ("scripts: Add a gen_release_notes.py script")
Reviewed-by: Eric Engestrom <eric@engestrom.ch>
Reviewed-by: Juan A. Suarez <jasuarez@igalia.com>
(cherry picked from commit 69f540c0)