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/milestone.go

@@ -54,10 +54,6 @@ var (
 		name:  projection.MilestoneColumnType,
 		table: milestonesTable,
 	}
-	MilestonePrimaryDomainColID = Column{
-		name:  projection.MilestoneColumnPrimaryDomain,
-		table: milestonesTable,
-	}
 	MilestoneReachedDateColID = Column{
 		name:  projection.MilestoneColumnReachedDate,
 		table: milestonesTable,
@@ -76,7 +72,10 @@ func (q *Queries) SearchMilestones(ctx context.Context, instanceIDs []string, qu
 	if len(instanceIDs) == 0 {
 		instanceIDs = []string{authz.GetInstance(ctx).InstanceID()}
 	}
-	stmt, args, err := queries.toQuery(query).Where(sq.Eq{MilestoneInstanceIDColID.identifier(): instanceIDs}).ToSql()
+	stmt, args, err := queries.toQuery(query).Where(
+		sq.Eq{MilestoneInstanceIDColID.identifier(): instanceIDs},
+		sq.Eq{InstanceDomainIsPrimaryCol.identifier(): true},
+	).ToSql()
 	if err != nil {
 		return nil, zerrors.ThrowInternal(err, "QUERY-A9i5k", "Errors.Query.SQLStatement")
 	}
@@ -96,13 +95,14 @@ func (q *Queries) SearchMilestones(ctx context.Context, instanceIDs []string, qu
 func prepareMilestonesQuery(ctx context.Context, db prepareDatabase) (sq.SelectBuilder, func(*sql.Rows) (*Milestones, error)) {
 	return sq.Select(
 			MilestoneInstanceIDColID.identifier(),
-			MilestonePrimaryDomainColID.identifier(),
+			InstanceDomainDomainCol.identifier(),
 			MilestoneReachedDateColID.identifier(),
 			MilestonePushedDateColID.identifier(),
 			MilestoneTypeColID.identifier(),
 			countColumn.identifier(),
 		).
 			From(milestonesTable.identifier() + db.Timetravel(call.Took(ctx))).
+			LeftJoin(join(InstanceDomainInstanceIDCol, MilestoneInstanceIDColID)).
 			PlaceholderFormat(sq.Dollar),
 		func(rows *sql.Rows) (*Milestones, error) {
 			milestones := make([]*Milestone, 0)