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/repository/sql/query.go

@@ -27,7 +27,7 @@ type querier interface {
 	eventQuery(useV1 bool) string
 	maxSequenceQuery(useV1 bool) string
 	instanceIDsQuery(useV1 bool) string
-	db() *database.DB
+	Client() *database.DB
 	orderByEventSequence(desc, shouldOrderBySequence, useV1 bool) string
 	dialect.Database
 }
@@ -110,7 +110,7 @@ func query(ctx context.Context, criteria querier, searchQuery *eventstore.Search
 	var contextQuerier interface {
 		QueryContext(context.Context, func(rows *sql.Rows) error, string, ...interface{}) error
 	}
-	contextQuerier = criteria.db()
+	contextQuerier = criteria.Client()
 	if q.Tx != nil {
 		contextQuerier = &tx{Tx: q.Tx}
 	}