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/mock/repository.mock.go

@@ -13,6 +13,7 @@ import (
 	context "context"
 	reflect "reflect"
 
+	database "github.com/zitadel/zitadel/internal/database"
 	eventstore "github.com/zitadel/zitadel/internal/eventstore"
 	gomock "go.uber.org/mock/gomock"
 )
@@ -40,6 +41,20 @@ func (m *MockQuerier) EXPECT() *MockQuerierMockRecorder {
 	return m.recorder
 }
 
+// Client mocks base method.
+func (m *MockQuerier) Client() *database.DB {
+	m.ctrl.T.Helper()
+	ret := m.ctrl.Call(m, "Client")
+	ret0, _ := ret[0].(*database.DB)
+	return ret0
+}
+
+// Client indicates an expected call of Client.
+func (mr *MockQuerierMockRecorder) Client() *gomock.Call {
+	mr.mock.ctrl.T.Helper()
+	return mr.mock.ctrl.RecordCallWithMethodType(mr.mock, "Client", reflect.TypeOf((*MockQuerier)(nil).Client))
+}
+
 // FilterToReducer mocks base method.
 func (m *MockQuerier) FilterToReducer(arg0 context.Context, arg1 *eventstore.SearchQueryBuilder, arg2 eventstore.Reducer) error {
 	m.ctrl.T.Helper()