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_test.go

@@ -11,13 +11,14 @@ import (
 
 var (
 	expectedMilestoneQuery = regexp.QuoteMeta(`
-		SELECT projections.milestones.instance_id,
-		   projections.milestones.primary_domain,
-		   projections.milestones.reached_date,
-		   projections.milestones.last_pushed_date,
-		   projections.milestones.type,
+		SELECT projections.milestones2.instance_id,
+		   projections.instance_domains.domain,
+		   projections.milestones2.reached_date,
+		   projections.milestones2.last_pushed_date,
+		   projections.milestones2.type,
 		   COUNT(*) OVER ()
-		FROM projections.milestones AS OF SYSTEM TIME '-1 ms'
+		FROM projections.milestones2 AS OF SYSTEM TIME '-1 ms'
+		LEFT JOIN projections.instance_domains ON projections.milestones2.instance_id = projections.instance_domains.instance_id
 		`)
 
 	milestoneCols = []string{