Fix fixing-template for matrix-alertmanager-receiver to also consider the `alertname` annotation

My alerts seem to contain `annotations.alertname` in the payload, so the
default configuration (coming from the matrix-alertmanager-receiver README)
seems to be outdated or something.

roles/custom/matrix-alertmanager-receiver/defaults/main.yml

@@ -205,7 +205,12 @@ matrix_alertmanager_receiver_config_templating_firing_template: |-
 # Controls the `templating.resolved-template` configuration setting.
 matrix_alertmanager_receiver_config_templating_resolved_template: |-
   {% raw %}
-  <strong><font color="{{ .ComputedValues.color }}">{{ .Alert.Status | ToUpper }}</font></strong>{{ .Alert.Labels.name }}
+  <strong><font color="{{ .ComputedValues.color }}">{{ .Alert.Status | ToUpper }}</font></strong>
+  {{ if .Alert.Labels.name }}
+    {{ .Alert.Labels.name }}
+  {{ else if .Alert.Labels.alertname }}
+    {{ .Alert.Labels.alertname }}
+  {{ end }}
   {% endraw %}
 
 # Default matrix-alertmanager-receiver configuration template which covers the generic use case.