"mbstring.func_overload" and "MB_OVERLOAD_STRING" removal from php8

[wesleyswordfish]

In preparartion for a php I noticed a problem

In the file byte_safe_strings.php the if statement check no longer seems necessary and these constants are no longer are defined in php8

[paragonie-security]

This polyfill doesn't do anything > PHP 7.

Set your requirements string to >= 2 and Composer should deliver a 9.99.99 version on PHP 8.