Enable new auto_closing field for major change

Author: Urgau

triagebot.toml

@@ -15,6 +15,7 @@ second_label = "final-comment-period"
 concerns_label = "has-concerns"
 accept_label = "major-change-accepted"
 waiting_period = 1
+auto_closing = true
 # can be found by looking for the first number in URLs, e.g. https://rust-lang.zulipchat.com/#narrow/stream/131828-t-compiler
 zulip_stream = 224082
 zulip_ping = "Urgau"