Link triagebot major-change handler to the has-concerns label

Author: Urgau

triagebot.toml

@@ -1,6 +1,7 @@
 [major-change]
 second_label = "final-comment-period"
 meeting_label = "to-announce"
+concerns_label = "has-concerns"
 open_extra_text = "cc @rust-lang/compiler"
 # 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 = 233931