Steps to reproduce the issue

Try to create a user with an apostrophe (single straight quote ' ), in the username part of the email like DeloresO'Grady@example.com

Expected result

Email is accepted.

Actual result

Email is declared invalid.

Additional comments

This was previously (in 2016) discussed here: #6982

And fixed here: #12835

But the email validation regex in media/system/fields/js/validate is currently set to not allow '

    this.setHandler('email', value => {
        const newValue = punycode.toASCII(value);
        const regex = /^[a-zA-Z0-9.!#$%&’*+/=?^_`{|}~-]+@[a-zA-Z0-9-]+(?:\.[a-zA-Z0-9-]+)*$/;
        return regex.test(newValue);
    });