* Cipher 1.9x speedup

Simplify cipher loop and unroll once.

```
λ benchcmp before.txt after.txt | grep -v "naive"
benchmark                                          old ns/op     new ns/op     delta
BenchmarkCipher/bytes=7;offset=1-32                7.17          7.79          +8.65%
BenchmarkCipher/bytes=125;offset=0-32              24.5          23.1          -6.07%
BenchmarkCipher/bytes=1024;offset=0-32             126           63.4          -49.87%
BenchmarkCipher/bytes=4096;offset=0-32             462           244           -47.26%
BenchmarkCipher/bytes=4100;offset=4-32             460           249           -45.93%
BenchmarkCipher/bytes=4099;offset=3-32             463           250           -45.93%
BenchmarkCipher/bytes=32775;offset=49-32           3619          1936          -46.50%

benchmark                                          old MB/s     new MB/s     speedup
BenchmarkCipher/bytes=7;offset=1-32                976.74       898.93       0.92x
BenchmarkCipher/bytes=125;offset=0-32              5092.84      5423.57      1.06x
BenchmarkCipher/bytes=1024;offset=0-32             8103.00      16159.29     1.99x
BenchmarkCipher/bytes=4096;offset=0-32             8870.86      16818.08     1.90x
BenchmarkCipher/bytes=4100;offset=4-32             8917.63      16491.32     1.85x
BenchmarkCipher/bytes=4099;offset=3-32             8854.38      16379.58     1.85x
BenchmarkCipher/bytes=32775;offset=49-32           9056.37      16926.83     1.87x
```

I tried a few variations, but this seemed fine without too many changes.

* Upgrade CI to more modern Go versions.

* Fix doc

* Revert "Upgrade CI to more modern Go versions."

This reverts commit ed29455.

Bumps [actions/checkout](https://github.com/actions/checkout) from 4 to 5.
- [Release notes](https://github.com/actions/checkout/releases)
- [Changelog](https://github.com/actions/checkout/blob/main/CHANGELOG.md)
- [Commits](actions/checkout@v4...v5)

---
updated-dependencies:
- dependency-name: actions/checkout
  dependency-version: '5'
  dependency-type: direct:production
  update-type: version-update:semver-major
...

Signed-off-by: dependabot[bot] <[email protected]>
Co-authored-by: dependabot[bot] <49699333+dependabot[bot]@users.noreply.github.com>

Bumps [actions/setup-go](https://github.com/actions/setup-go) from 5 to 6.
- [Release notes](https://github.com/actions/setup-go/releases)
- [Commits](actions/setup-go@v5...v6)

---
updated-dependencies:
- dependency-name: actions/setup-go
  dependency-version: '6'
  dependency-type: direct:production
  update-type: version-update:semver-major
...

Signed-off-by: dependabot[bot] <[email protected]>
Co-authored-by: dependabot[bot] <49699333+dependabot[bot]@users.noreply.github.com>

Bumps [actions/upload-artifact](https://github.com/actions/upload-artifact) from 4 to 5.
- [Release notes](https://github.com/actions/upload-artifact/releases)
- [Commits](actions/upload-artifact@v4...v5)

---
updated-dependencies:
- dependency-name: actions/upload-artifact
  dependency-version: '5'
  dependency-type: direct:production
  update-type: version-update:semver-major
...

Signed-off-by: dependabot[bot] <[email protected]>
Co-authored-by: dependabot[bot] <49699333+dependabot[bot]@users.noreply.github.com>