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/eventstore/eventstore.go

@@ -11,6 +11,7 @@ import (
 	"github.com/zitadel/logging"
 
 	"github.com/zitadel/zitadel/internal/api/authz"
+	"github.com/zitadel/zitadel/internal/database"
 	"github.com/zitadel/zitadel/internal/zerrors"
 )
 
@@ -247,6 +248,10 @@ func (es *Eventstore) InstanceIDs(ctx context.Context, maxAge time.Duration, for
 	return instances, nil
 }
 
+func (es *Eventstore) Client() *database.DB {
+	return es.querier.Client()
+}
+
 type QueryReducer interface {
 	reducer
 	//Query returns the SearchQueryFactory for the events needed in reducer
@@ -270,6 +275,8 @@ type Querier interface {
 	LatestSequence(ctx context.Context, queryFactory *SearchQueryBuilder) (float64, error)
 	// InstanceIDs returns the instance ids found by the search query
 	InstanceIDs(ctx context.Context, queryFactory *SearchQueryBuilder) ([]string, error)
+	// Client returns the underlying database connection
+	Client() *database.DB
 }
 
 type Pusher interface {