Colors: remove deprecated isBright and isDark functions

[mrtnzlml]

The goal of this task is to remove deprecated functions located in src/css-colors/src/isBright.js and src/css-colors/src/isDark.js, respectively. This should include removing of all related files and updating the README file and CHANGELOG (all located under src/css-colors/).

Related issue: #2908

Acceptance criteria

1. The isBright and isDark functions and all related files were removed and cleaned up.
2. README and CHANGELOG located in src/css-colors/ are updated to reflect the new reality.
3. The PR follows the project conventions.
4. All CI checks pass, and all code review comments are addressed.

How to install and run this project

git clone --depth=100 [email protected]:adeira/universe.git
cd universe
corepack enable

chmod +x ./x
./x install

To run JavaScript tests:

./x tests

For more information, please visit README.md located at the root of this project. If you get lost, you can also start a new discussion. 😎

[mrtnzlml]

Successfully completed in #5106, thanks @jaebirds!