Bumps the optional `zstd` dependency from `0.13` to `0.14` (pulls in
`zstd-safe` 8.0.0 / `zstd-sys` 2.1.0).
We only use `zstd::Decoder::new` for streaming decode, which is unchanged
in 0.14.0, so this is a pure version bump with no code changes.
Also worth noting: `zstd-safe` 8.0.0 fixes a dangling dictionary pointer
reachable from safe code, and `zstd-sys` 2.1.0 no longer needs `libclang`
by default. The crate stack moved from MIT to BSD-3-Clause (still
permissive, compatible with our MIT).
