Use "reason" field to explain ignored advisory

This turns the comment in `cargo.deny` explaining why we currently
ignore the informational advisory RUSTSEC-2024-0436 in `paste` into
data that could be parsed and displayed by tools, by using the form
with `id` and `reason` fields, where the old comment text is the
value of the `reason` field.

This is one of the forms documented in:
https://embarkstudios.github.io/cargo-deny/checks/advisories/cfg.html#the-ignore-field-optional

Author: EliahKagan

deny.toml

@@ -9,8 +9,7 @@
 # https://embarkstudios.github.io/cargo-deny/checks/advisories/cfg.html
 [advisories]
 ignore = [
-    # `paste` - macro crate without replacement
-    "RUSTSEC-2024-0436"
+    { id = "RUSTSEC-2024-0436", reason = "`paste` - macro crate without replacement" },
 ]