docs: drop the pre-release README note after 4.0.0

[damwilin]

Summary

Post-release cleanup for 4.0.0:

• README.md: removed the "TODO before tagging 4.0.0" comment at the top; the README now describes what is on Maven Central.
• Demo app bumped to 1.28 (24) for the internal test build of the released code.

The changelog itself was already dated on main (a44e387), so this PR leaves it as is.

Type of Change

• Bug fix
• Feature
• Documentation
• Refactor or cleanup
• CI/build/release
• Breaking change

Affected Areas

• API module
• Player module
• Camera recording/upload module
• Android TV module
• Demo app
• Documentation
• Build or publishing

Testing

• ./gradlew testDebugUnitTest --no-daemon --stacktrace
• ./gradlew :app:assembleDebug --no-daemon
• Manual playback/upload/recording test
• Not run, reason: documentation and demo version only

SDK User Impact

None. Documentation only; the published 4.0.0 artifacts are unchanged.

AI Assistance

• No substantial AI assistance was used.
• AI helped with this PR; I reviewed and understand the final changes.

Checklist

• The PR is focused on one clear change.
• Public API or behavior changes are documented.
• Tests or manual verification are included where appropriate.
• No secrets, access keys, or private data are included.