fix(milestones): offset the type enum (#8849)

# Which Problems Are Solved

Migration of milestones failed on our QA due to the new milestone Type
enum being 0-indexed. The valid range was 0 till 5, inclusive. While on
the previous zitadel version this was 1 till 6, inclusive.

# How the Problems Are Solved

Offset the first constant with `1`.

# Additional Changes

- none

# Additional Context

Introduced in https://github.com/zitadel/zitadel/pull/8788

internal/repository/milestone/events.go

@@ -11,7 +11,7 @@ import (
 type Type int
 
 const (
-	InstanceCreated Type = iota
+	InstanceCreated Type = iota + 1
 	AuthenticationSucceededOnInstance
 	ProjectCreated
 	ApplicationCreated