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/query/projection/milestones.go

@@ -10,7 +10,7 @@ import (
 )
 
 const (
-	MilestonesProjectionTable = "projections.milestones2"
+	MilestonesProjectionTable = "projections.milestones3"
 
 	MilestoneColumnInstanceID  = "instance_id"
 	MilestoneColumnType        = "type"
@@ -77,9 +77,6 @@ func (p *milestoneProjection) reducePushed(event eventstore.Event) (*handler.Sta
 	if err != nil {
 		return nil, err
 	}
-	if e.Agg.Version != milestone.AggregateVersion {
-		return handler.NewNoOpStatement(event), nil // Skip v1 events.
-	}
 	if e.MilestoneType != milestone.InstanceDeleted {
 		return handler.NewUpdateStatement(
 			event,