perf(milestones): refactor (#8788)

# Which Problems Are Solved

Milestones used existing events from a number of aggregates. OIDC
session is one of them. We noticed in load-tests that the reduction of
the oidc_session.added event into the milestone projection is a costly
business with payload based conditionals. A milestone is reached once,
but even then we remain subscribed to the OIDC events. This requires the
projections.current_states to be updated continuously.


# How the Problems Are Solved

The milestone creation is refactored to use dedicated events instead.
The command side decides when a milestone is reached and creates the
reached event once for each milestone when required.

# Additional Changes

In order to prevent reached milestones being created twice, a migration
script is provided. When the old `projections.milestones` table exist,
the state is read from there and `v2` milestone aggregate events are
created, with the original reached and pushed dates.

# Additional Context

- Closes https://github.com/zitadel/zitadel/issues/8800

internal/repository/milestone/aggregate.go

@@ -9,20 +9,23 @@ import (
 
 const (
 	AggregateType    = "milestone"
-	AggregateVersion = "v1"
+	AggregateVersion = "v2"
 )
 
 type Aggregate struct {
 	eventstore.Aggregate
 }
 
-func NewAggregate(ctx context.Context, id string) *Aggregate {
-	instanceID := authz.GetInstance(ctx).InstanceID()
+func NewAggregate(ctx context.Context) *Aggregate {
+	return NewInstanceAggregate(authz.GetInstance(ctx).InstanceID())
+}
+
+func NewInstanceAggregate(instanceID string) *Aggregate {
 	return &Aggregate{
 		Aggregate: eventstore.Aggregate{
 			Type:          AggregateType,
 			Version:       AggregateVersion,
-			ID:            id,
+			ID:            instanceID,
 			ResourceOwner: instanceID,
 			InstanceID:    instanceID,
 		},