updated Unicode LICENSE, README.md, CONTRIBUTING.md

- Updated license: https://www.unicode.org/license.txt
   - Want a **diff** of the LICENSE file? go [here](https://github.com/unicode-org/icu/pull/2552#discussion_r1298730343) for a diff!
- Updated README.md
- Updated [CONTRIBUTING.md](https://github.com/unicode-org/.github/blob/main/.github/CONTRIBUTING.md)
