Fix unused variable in json_decode_dict_key under FT builds - #1145

The 0.21.1 to 0.22.0 changelog batch (msgspec#1112) landed on 2026-07-03. Four
user-visible changes merged between then and the 0.22.0 release on
2026-08-11 never got an entry:

- msgspec#700, overloads on the `Meta` stub, so type checkers reject mixing
`gt` with `ge` and `lt` with `le`
- msgspec#1028, `null` placed last in the `anyOf` generated for optional unions
- msgspec#1114, `__struct_encode_fields__` added to `src/msgspec/__init__.pyi`
(its companion msgspec#813 only touched `tests/typing`, so it needs no entry of
its own)
- msgspec#1118, `json.encode` raising `TypeError` for a `dict` keyed by a plain
`str`-valued `enum.Enum`

I went over every PR merged in that window. The rest need no entry: msgspec#813
and msgspec#1117 are tests only, msgspec#1121 fixes an unreleased regression from
msgspec#1028, msgspec#1145 is an unused variable under free-threaded builds, and msgspec#1146
and msgspec#1148 are CI and tooling. msgspec#1127, msgspec#1135 and msgspec#1080 already have
entries.

Docs build clean with `--fail-on-warning`.

Note for whoever retries the release: the section is still headed
`Version 0.22.0 (2026-08-11)`, and that date will need updating, since
the tag was deleted after the upload failed. Refs msgspec#1134.

Co-authored-by: Siyet <[email protected]>