[MIG] *: fix error object has no attribute 'render_template'

Reviewed ir.actions.report and mail template models; Replace
obsolete methode "render_template" by "_render_template"

models/ir_actions_report.py

@@ -141,7 +141,7 @@
         Template = self.env["mail.template"].with_context(safe=True)
 
         for sub in self.substitution_ids:
-            value = Template.render_template(sub.value, self.model, res_id)
+            value = Template._render_template(sub.value, self.model, res_id)
             if sub.deserialize:
                 value = ast.literal_eval(value)
             values_sent_to_redner[sub.keyword] = value

models/mail_template.py

@@ -60,7 +60,7 @@
         # get redner template values
         values_sent_to_redner = {}
         for sub in self.redner_substitution_ids:
-            value = self.render_template(sub.value, self.model, res_id)
+            value = self._render_template(sub.value, self.model, res_id)
             if sub.deserialize:
                 value = ast.literal_eval(value)
             values_sent_to_redner[sub.keyword] = value