docs: update ESLint documentation with ESLint v9 (#9515)
### Description
ESLint v9 is now the standard, with ESLint v8 past EOL as of October 5,
2024.
This PR updates our docs to describe how to use ESLint v9 Flat Configs
in monorepos.
Additionally, the READMEs for the packages are updated in this PR.
