rustbot: Allow applying relnotes label

Inspired by rust-lang#90521 , and by the
various `must_use` PRs; in all of those cases, the submitter of the PR
could know that `relnotes` applied, but couldn't apply it themselves.

triagebot.toml

@@ -2,6 +2,7 @@
 allow-unauthenticated = [
     "C-*", "A-*", "E-*", "NLL-*", "O-*", "S-*", "T-*", "WG-*", "F-*",
     "D-*",
+    "relnotes",
     "requires-nightly",
     "regression-*",
     "perf-*",