Update Changelog for 0.76.8 (#50296)

Summary:
Update changelog for the 0.76.8 release:

[All commits: https://github.com/facebook/react-native/compare/v0.77.1...0.77-stable](https://github.com/facebook/react-native/compare/v0.76.7...v0.76.8)

## Changelog:

[INTERNAL] [ADDED] - Changelog for 0.76.8

Pull Request resolved: https://github.com/facebook/react-native/pull/50296

Test Plan: N/A

Reviewed By: shwanton

Differential Revision: D71911064

Pulled By: arushikesarwani94

fbshipit-source-id: 515f48d0cf6525c56024e5791b45a3c1f8a12186