[FIX] account_peppol: incorrect view extension

`account_peppol` hooks its addition of `option_peppol` in the invoice
sending form to `option_send_by_post`.

However `option_send_by_post` is added by `snailmail_account` which
`account_peppol` does *not* depend on. And while it's likely
`snailmail_account` has long been installed when `account_peppol` gets
installed there's no actual guarantee, and `snailmail` could even have
been uninstalled.

Which is exactly the issue, when uninstalling `snailmail_account` or
any of its dependencies which don't uninstall `account_peppol`
(`snailmail`, `iap_mail`, `iap`) the "send & print" (send invoice)
wizard is broken.

Fix by re-hooking the view extension on `option_send_mail` instead,
that is installed by an actual dependency of `account_peppol`, and
part of the view which `account_peppol` actually extends
(`account.account_move_send_form`).

closes odoo/odoo#121522

Related: odoo/enterprise#41124
Signed-off-by: Xavier Morel (xmo) <xmo@odoo.com>

addons/account_peppol/wizard/account_move_send_views.xml

@@ -17,7 +17,7 @@
                         <field name="peppol_warning"/>
                     </div>
                 </xpath>
-                <xpath expr="//div[@name='option_send_by_post']" position='before'>
+                <xpath expr="//div[@name='option_send_mail']" position='after'>
                     <div name="option_peppol"
                          attrs="{'invisible': ['|', ('peppol_proxy_state', '!=', 'active'), ('enable_ubl_cii_xml', '=', False)]}">
                         <field name="checkbox_send_peppol" attrs="{'readonly': [('checkbox_ubl_cii_xml', '=', False)]}"/>