[FIX] portal: Portal User creation login check

- When creating a user from the "Grant Portal Access" wizard, the code
  checks for duplicate users.
  It does so by checking if there is a user with the same login as the
  one we want to create.

  The check is case sensitive which could lead to issues.
  For example a mistyped email (with a uppercase somewhere) could fail
  the duplicate user check.
  Since emails are not case sensitive, we want the check to be case
  insensitive too.

  OPW-1932918

closes odoo/odoo#31818

Signed-off-by: Toufik Benjaa (tbe) <tbe@odoo.com>

addons/portal/wizard/portal_wizard.py

@@ -93,7 +93,7 @@ class PortalWizardUser(models.TransientModel):
                 partners_error_empty |= wizard_user.partner_id
             elif email in emails:
                 partners_error_emails |= wizard_user.partner_id
-            user = self.env['res.users'].sudo().with_context(active_test=False).search([('login', '=', email)])
+            user = self.env['res.users'].sudo().with_context(active_test=False).search([('login', '=ilike', email)])
             if user:
                 partners_error_user |= wizard_user.partner_id
             emails.append(email)