The `typos` CI action recently started complaining about these:

https://github.com/RustCrypto/block-ciphers/actions/runs/18258797158/job/51983873161?pr=505

See also: RustCrypto/hashes#741

The former has been removed from the latest `nightly` compiler releases

Also removes the `--cfg docsrs` settings, since they're now automatic

See also: RustCrypto/traits#2028

Closes #504

This is #491, rebased on `master`.

Closes #489

Co-authored-by: silvanshade <silvanshade@users.noreply.github.com>

Implementation and documentation are aligned with rc5 implementation.

I referred following document for key schedule, encryption and
decryption algorithms:

https://www.grc.com/r&d/rc6.pdf

I checked whether it works by following test vectors.

https://datatracker.ietf.org/doc/html/draft-krovetz-rc6-rc5-vectors-00#section-3

Updates `rc6` to use the latest Rust version and version of `cipher`
that all of the other crates are using

Cuts the following prereleases:

- `aes` v0.9.0-rc.2
- `belt-block` v0.2.0-rc.2
- `blowfish` v0.10.0-rc.2
- `des` v0.9.0-rc.2
- `gift-cipher` v0.1.0-rc.2
- `kuznyechik` v0.9.0-rc.2
- `magma` v0.10.0-rc.2
- `rc6` v0.1.0-rc.2
- `serpent` v0.6.0-rc.2
- `sm4` v0.6.0-rc.2
- `speck-cipher` v0.1.0-rc.2