http4k: `HmacSha256.hash` (despite the `Hmac` naming) computed a plain unkeyed digest; clarified by deprecation in favour of `Sha256.hash` / `Sha256.hmac`

Summary

### Impact

The `HmacSha256` class contained two functions: - `hash(payload)` — a plain unkeyed SHA-256 digest. The `Hmac` prefix in the class name was misleading; this function has no key parameter, so it could never have been an HMAC. - `hmacSHA256(key, data)` — a properly keyed HMAC-SHA256.

A reader who didn't engage with the function signature could in principle have assumed `HmacSha256.hash(payload)` was somehow keyed, but the absence of any key parameter made that misuse unlikely in practice.

**Who is affected:** any downstream caller who read the class name and used `HmacSha256.hash` as a message authentication code without noticing it takes no key. **Verified at v6.47.2.0: zero internal misuse in http4k itself.** Both production usages of `HmacSha256.hash` (AWS SigV4 canonical-request hashing in `AwsSignatureV4Signer.kt` and `x-amz-content-sha256` in `awsExtensions.kt`) are AWS-spec-correct uses of plain SHA-256; every keyed `hmacSHA256(key, data)` call passes a real key. The advisory exists so any downstream caller relying on the misleadingly-named API knows to migrate.

### Patches

Upgrade to **6.49.0.0** or later. The fix introduces: - `Sha256.hash(input)` — unkeyed digest (the actual behaviour `HmacSha256.hash` provided). - `Sha256.hmac(key, input)` — keyed HMAC-SHA256 (the behaviour the name implied).

`HmacSha256` is deprecated. Existing callers continue to work via deprecation shims; migrate to `Sha256.hash` or `Sha256.hmac` per intent.

### Workarounds

If you cannot upgrade and you need a real HMAC-SHA256, use `javax.crypto.Mac.getInstance("HmacSHA256")` with a `SecretKeySpec`. For an unkeyed SHA-256 digest, use `java.security.MessageDigest.getInstance("SHA-256")`. The keyed `hmacSHA256(key, data)` was always correctly implemented and is safe to use as-is.

### References

- Fix release: [v6.49.0.0](https://github.com/http4k/http4k/releases/tag/6.49.0.0) - Background: [RFC 2104 — HMAC: Keyed-Hashing for Message Authentication](https://datatracker.ietf.org/doc/html/rfc2104)