chore: release alkhttp 0.3.0

- version bump 0.2.0 -> 0.3.0 (the alkcall 0.6.0 consumption wave
  carries a compile-breaking change for downstream users of
  `OpenableAlpn::new`: handler closures gain the `_plan` parameter,
  so this release cannot be a patch)
- README usage snippet 0.2 -> 0.3
- changelog: cut [0.3.0] with the alkcall 0.6.0 wave entry
  (OpenHandler plan parameter, Establishment.plan threading,
  pump_bidi availability)

Verification: cargo test (454 passed), cargo test --all-features
(587 passed), clippy -D warnings (default + all-features),
cargo fmt --check, cargo doc --no-deps,
cargo publish --dry-run --allow-dirty

README.md

@@ -161,7 +161,7 @@ A lean single-side build takes `default-features = false` plus the side
 it needs:
 
 ```toml
-alkhttp = { version = "0.2", default-features = false, features = ["server"] }
+alkhttp = { version = "0.3", default-features = false, features = ["server"] }
 ```
 
 ## Security posture