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/eventstore.go

@@ -4,6 +4,12 @@ import (
 	"github.com/zitadel/zitadel/internal/eventstore"
 )
 
+var (
+	ReachedEventMapper = eventstore.GenericEventMapper[ReachedEvent]
+	PushedEventMapper  = eventstore.GenericEventMapper[PushedEvent]
+)
+
 func init() {
+	eventstore.RegisterFilterEventMapper(AggregateType, ReachedEventType, ReachedEventMapper)
 	eventstore.RegisterFilterEventMapper(AggregateType, PushedEventType, PushedEventMapper)
 }