Prepare v6 release

[amyu]

Summary

• prepare action metadata and cache namespacing for v6
• publish exact release tags only and stop maintaining a floating major-version tag
• update the default SDK and CI examples to current SDK, Build Tools, NDK, and CMake versions
• retain codename-based SDK platform support with availability guidance and a stable negative test
• replace the legacy ndk-bundle PATH entry with the selected side-by-side NDK path
• refresh the documentation, license years, sample Android project, and bundled dist files

Why

Version 6 changes the release policy so consumers select a full release tag such as v6.0 instead of a floating major tag. This is also a good boundary for removing stale Android configuration and documentation while preserving support for codename packages whenever Google publishes them.

Impact

• release runs create or update only the requested full version tag
• the v6 cache namespace is isolated from v5
• ndk-version adds $ANDROID_SDK_ROOT/ndk/<version> to PATH
• sdk-version accepts numeric platform suffixes and currently published codename suffixes
• the default SDK version becomes 37.0

Validation

• pnpm run all
• actionlint
• ./sample-android-project/gradlew -p sample-android-project help --warning-mode all
• git diff --check