chore: Correct Alert Method type names (#476)

## Motivation

Having sth like `v1alpha.Email` in the namespace is way too vague, we
need more specific naming, that's why we need a prefix.

manifest/v1alpha/alert_methods.go

@@ -1,8 +1,8 @@
 package v1alpha
 
-//go:generate ../../bin/go-enum  --values --noprefix
+//go:generate ../../bin/go-enum  --values
 
-// DataSourceType represents the specific type of alert method.
+// AlertMethodType represents the specific type of alert method.
 //
 /* ENUM(
 Webhook = 1

manifest/v1alpha/alertmethod/alert_method.go

@@ -62,23 +62,23 @@ type Spec struct {
 func (s Spec) GetType() (v1alpha.AlertMethodType, error) {
 	switch {
 	case s.Webhook != nil:
-		return v1alpha.Webhook, nil
+		return v1alpha.AlertMethodTypeWebhook, nil
 	case s.PagerDuty != nil:
-		return v1alpha.PagerDuty, nil
+		return v1alpha.AlertMethodTypePagerDuty, nil
 	case s.Slack != nil:
-		return v1alpha.Slack, nil
+		return v1alpha.AlertMethodTypeSlack, nil
 	case s.Discord != nil:
-		return v1alpha.Discord, nil
+		return v1alpha.AlertMethodTypeDiscord, nil
 	case s.Opsgenie != nil:
-		return v1alpha.Opsgenie, nil
+		return v1alpha.AlertMethodTypeOpsgenie, nil
 	case s.ServiceNow != nil:
-		return v1alpha.ServiceNow, nil
+		return v1alpha.AlertMethodTypeServiceNow, nil
 	case s.Jira != nil:
-		return v1alpha.Jira, nil
+		return v1alpha.AlertMethodTypeJira, nil
 	case s.Teams != nil:
-		return v1alpha.Teams, nil
+		return v1alpha.AlertMethodTypeTeams, nil
 	case s.Email != nil:
-		return v1alpha.Email, nil
+		return v1alpha.AlertMethodTypeEmail, nil
 	}
 	return 0, errors.New("unknown alert method type")
 }