Upgrade Netty to 4.2.17

[NianJiuZst]

Summary

• upgrade Netty dependencies from 4.2.16.Final to 4.2.17.Final in the Gradle and Bazel declarations
• document the Netty version used by the 1.85 development line
• retain netty-tcnative 2.0.81.Final, which is the version used by Netty 4.2.17

Netty 4.2.17 is the first 4.2 release patched for CVE-2026-75596.

Testing

• ./gradlew :grpc-netty:assemble :grpc-netty:checkstyleMain :grpc-netty:checkstyleTest :grpc-netty:animalsnifferMain :grpc-netty-shaded:assemble -PskipCodegen=true -PskipAndroid=true
• ./gradlew :grpc-netty:build :grpc-netty-shaded:build -PskipCodegen=true -PskipAndroid=true: 647 Netty tests completed; 2 timed out and 79 were skipped. The same two NettyServerTest timeouts, multiPortConnections and connectionSettingsPropagated, reproduce on an unmodified worktree at upstream/master fc4314419dca8dab8383f8687c9be4982dae9054.

Fixes #13027