upgrades: deflake TestRoleMembersIDMigration1500Users

TeamCity has a new machine type where this test has started to time out
more, so this change will make it take less time.

Release note: None

pkg/upgrade/upgrades/role_members_ids_migration_test.go

@@ -36,10 +36,10 @@ func TestRoleMembersIDMigration10Users(t *testing.T) {
 	runTestRoleMembersIDMigration(t, 10)
 }
 
-func TestRoleMembersIDMigration1500Users(t *testing.T) {
+func TestRoleMembersIDMigration1000Users(t *testing.T) {
 	skip.UnderRace(t)
 	skip.UnderStress(t)
-	runTestRoleMembersIDMigration(t, 1500)
+	runTestRoleMembersIDMigration(t, 1000)
 }
 
 func runTestRoleMembersIDMigration(t *testing.T, numUsers int) {