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/cache/cache.go

@@ -6,6 +6,7 @@ import (
 	"time"
 
 	"github.com/zitadel/logging"
+
 	"github.com/zitadel/zitadel/internal/database/postgres"
 )
 
@@ -77,7 +78,8 @@ type CachesConfig struct {
 		Postgres PostgresConnectorConfig
 		// Redis redis.Config?
 	}
-	Instance *CacheConfig
+	Instance   *CacheConfig
+	Milestones *CacheConfig
 }
 
 type CacheConfig struct {