[FIX] core: add UPDATE as valid many2many command in SSF

This commit fix the "Unsupported M2M command 1" raised `Form` helper.

The `onchange()` method will in fact emit UPDATE command for many2many
fields when the value submitted an the one in database has changed
(this is the case for example for nested m2m in form views)

OPW-2044631

closes odoo/odoo#59937

X-original-commit: 76bd8208a416cefab6becff9f2d7204c7d196201
Signed-off-by: Xavier Morel (xmo) <xmo@odoo.com>
Signed-off-by: Xavier ALT <xavieralt@users.noreply.github.com>