[FIX] website_form: fix form ids generation

In order to respect the good practices of HTML, it is preferable that
the IDs of HTML elements do not start with a number. This makes it
easier to handle CSS selectors etc.

task-2760205

closes odoo/odoo#84236

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