fix(milestones): use previous spelling for milestone types (#8886)

# Which Problems Are Solved

https://github.com/zitadel/zitadel/pull/8788 accidentally changed the
spelling of milestone types from PascalCase to snake_case. This breaks
systems where `milestone.pushed` events already exist.

# How the Problems Are Solved

- Use PascalCase again
- Prefix event types with v2. (Previous pushed event type was anyway
ignored).
- Create `milstones3` projection

# Additional Changes

None

# Additional Context

relates to #8788

internal/repository/milestone/events.go

@@ -7,7 +7,7 @@ import (
 	"github.com/zitadel/zitadel/internal/eventstore"
 )
 
-//go:generate enumer -type Type -json -linecomment -transform=snake
+//go:generate enumer -type Type -json -linecomment
 type Type int
 
 const (
@@ -20,7 +20,7 @@ const (
 )
 
 const (
-	eventTypePrefix  = "milestone."
+	eventTypePrefix  = "milestone.v2."
 	ReachedEventType = eventTypePrefix + "reached"
 	PushedEventType  = eventTypePrefix + "pushed"
 )