v2.9.0 (#1139)

## Summary

Release 2.9.0 — consumes all 20 pending changesets (8 minor / 12 patch →
minor bump), matching the published `v2.9.0-dev.0` pre-release line.

### Highlights
- **Added**: certificate pinning, RPC v2, audio engine availability +
`externalCallSystem` mode for CallKit, AudioManager session/routing
APIs, ConnectionCheck utility, microphone mute mode, runtime audio
processing controls, desktop audio frame capture, VideoTrackRenderer
placeholder
- **Fixed**: removal of the non-public `buttonPressed:` selector (App
Store rejection risk), Android media routing / `selectAudioOutput`,
audio processing lifecycle fixes, macOS platform video rendering, single
disconnected event, video render mode switching

### Release bookkeeping / repo organization
- Version bumped to 2.9.0 in `.version`, `pubspec.yaml`, both podspecs,
`lib/src/livekit.dart`, and README
- **Backfills the missing `2.8.1` CHANGELOG section** — 2.8.1 was
released from its branch without merging bookkeeping back, leaving main
at 2.8.0 with two already-released changesets still pending; this PR
reconciles that (the 2.9.0 section excludes the two entries that shipped
in 2.8.1)
- Cherry-picks the `docs/` → `doc/` pub layout chore that was stranded
on `release/2.9.0-dev.0`
- Declares the certificate-pinning test fixture key in `false_secrets` —
without this, pub.dev **rejects the publish** (validated via dry-run)

## Verification

- `dart pub publish --dry-run` passes (only the intentional
`flutter_webrtc` exact-pin warning remains, same as prior releases)
- `flutter analyze` clean on the flutter-callkit example against this
branch (only expected `@experimental` warnings)
- No stale `2.8.0` version references remain

## After merge

1. Tag `v2.9.0` on main → publish workflow pushes to pub.dev
2. Delete `release/2.9.0-dev.0` (superseded by the stable release)

Note: the changeset-check workflow will warn "No changeset found" —
expected for a release PR that consumes `.changes/`.

Author: hiroshihorie