Add PHP 8 as an allowed version in composer.json #5475

From my testing so far, it looks like WP CLI is compatible with PHP 8 for the most part. After fixing those issues, it'd be great to add PHP 8 to the required PHP versions list in composer.json so WP CLI can be installed with composer in those systems without using --ignore-platform-reqs.

Found issues: wp-cli/search-replace-command#147, #5472, wp-cli/profile-command#162.