spanner: allow spicedb to run with head or head-1 migration

spanner has a 2 phase migration for the last change - spicedb should
be able to run at the first (adding the new changestream) and the latest
(removing the old) without problems. Prior to this change, the DS health
check returned an error if not at the latest schema change.

internal/datastore/spanner/spanner.go

@@ -259,8 +259,8 @@ func (sd spannerDatastore) ReadyState(ctx context.Context) (datastore.ReadyState
 		return datastore.ReadyState{}, err
 	}
 
-	// TODO(jschorr): Remove register-tuple-change-stream once the multi-phase is done.
-	if version == headMigration || version == "register-tuple-change-stream" {
+	// TODO: once phased migration is complete, remove the extra allowed version
+	if version == headMigration || version == "register-combined-change-stream" {
 		return datastore.ReadyState{IsReady: true}, nil
 	}