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/query/projection/event_test.go

@@ -14,8 +14,9 @@ func testEvent(
 	eventType eventstore.EventType,
 	aggregateType eventstore.AggregateType,
 	data []byte,
+	opts ...eventOption,
 ) *repository.Event {
-	return timedTestEvent(eventType, aggregateType, data, time.Now())
+	return timedTestEvent(eventType, aggregateType, data, time.Now(), opts...)
 }
 
 func toSystemEvent(event *repository.Event) *repository.Event {
@@ -28,8 +29,9 @@ func timedTestEvent(
 	aggregateType eventstore.AggregateType,
 	data []byte,
 	creationDate time.Time,
+	opts ...eventOption,
 ) *repository.Event {
-	return &repository.Event{
+	e := &repository.Event{
 		Seq:           15,
 		CreationDate:  creationDate,
 		Typ:           eventType,
@@ -42,6 +44,18 @@ func timedTestEvent(
 		ID:            "event-id",
 		EditorUser:    "editor-user",
 	}
+	for _, opt := range opts {
+		opt(e)
+	}
+	return e
+}
+
+type eventOption func(e *repository.Event)
+
+func withVersion(v eventstore.Version) eventOption {
+	return func(e *repository.Event) {
+		e.Version = v
+	}
 }
 
 func baseEvent(*testing.T) eventstore.Event {