Auto merge of #11498 - dtolnay-contrib:relabel, r=ehuss

Enable triagebot's relabel functionality

### What does this PR try to resolve?

This fixes the following failure that rustbot currently posts whenever someone tries to use "<b>`@</b><b>rustbot</b>` label" in this repository.

> **Error**: The feature `relabel` is not enabled in this repository.
> To enable it add its section in the `triagebot.toml` in the root of the repository.

Unauthenticated relabel has been enabled in rust-lang/rust for nearly 4 years. People overwhelmingly use it in good faith.

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### How should we test and review this PR?

Compare against https://github.com/rust-lang/rust/blob/1.66.0/triagebot.toml.

Also skim through the 7 pages of labels on https://github.com/rust-lang/cargo/labels, whether it makes sense the ones I decided to allow arbitrary GitHub users to apply.

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### Additional information

Attempted uses of "<b>`@</b><b>rustbot</b>` label", that failed, but this PR would allow:

- #10343 (comment)
- #10243 (comment)
- #9982 (comment)
- #9128 (comment)
- #9067 (comment)
- #8441 (comment)
- #11432 (comment)
- #8841 (comment)
- #10820 (comment)
- #10572 (comment)
- #9114 (comment)
- #8980 (comment)
- #9064 (comment)
- #8726 (comment)
- #8089 (comment)

triagebot.toml

@@ -1,3 +1,17 @@
+[relabel]
+allow-unauthenticated = [
+    "A-*",
+    "C-*",
+    "Command-*",
+    "E-*",
+    "O-*",
+    "S-*",
+    "Z-*",
+    "needs-mcve",
+    "regression-*",
+    "relnotes",
+]
+
 [ping.windows]
 message = """\
 Hey Windows Group! This bug has been identified as a good "Windows candidate".