docs: update README for ES2023

[sosukesuzuki]

Follow up #556

[eslint-github-bot]

Hi @sosukesuzuki!, thanks for the Pull Request

The first commit message isn't properly formatted. We ask that you update the message to match this format, as we use it to generate changelogs and automate releases.

• The commit message tag wasn't recognized. Did you mean "docs", "fix", or "feat"?
• There should be a space following the initial tag and colon, for example 'feat: Message'.
• The first letter of the tag should be in lowercase

To Fix: You can fix this problem by running git commit --amend, editing your commit message, and then running git push -f to update this pull request.

Read more about contributing to ESLint here

[mdjermanovic]

LGTM, thanks!