[FIX] website: update comment in user_custom_bootstrap_overridden file

The comment became outdated:
- For 14.0 and above: mentioned the "Customize dialog" which does not
  exist anymore.
- For 16.0 and above: mentioned "Bootstrap 4" instead of "Bootstrap 5".

Related to opw-3056683

closes odoo/odoo#106794

Signed-off-by: Quentin Smetz (qsm) <qsm@odoo.com>