lower MSRV to 1.90, trim two stray comments in aacs handshake

Bisected the true floor: 1.85 fails to compile (needs is_multiple_of,
stabilized 1.87, and let-chains, stabilized 1.88), 1.90 builds/clippy/tests
clean. Repin Cargo.toml rust-version and every CI dtolnay/rust-toolchain to
1.90.0, and note the change in CHANGELOG.md. Also removed a redundant
"// Hash the data" comment and a stale duplicate doc line on
generate_host_key_pair_p256 found during a pass over the crate's comments.

CHANGELOG.md

@@ -10,6 +10,9 @@
   discontinuity, instead of splicing post-gap audio mid-stream — fixing
   decoder-choking seams on discs whose stream carries a continuity-counter gap;
   see the libfreemkv 1.6.10 notes).
+- MSRV lowered from 1.97 to 1.90. The crate does not use anything newer than
+  1.90 stabilized; CI's pinned toolchain and `rust-version` are updated to
+  match.
 
 ## [1.6.9] — 2026-08-22