Can't revoke super admin if email is different case from network admin

[trepmal]

In multisite, if a Super Admin's email is a case-insensitive match to the Network Admin email, the checkbox for revoking super-admin shows, but doesn't work.

Reproduction:

• set Network Admin email to Admin@example.test
• set a Super Admin's email to admin@example.test
• attempt to revoke super admin from same user

This will fail because of the case-insensitive comparison here:
https://core.trac.wordpress.org/browser/trunk/src/wp-includes/capabilities.php#L922

In typical circumstances (when emails do not have a case difference) the checkbox doesn't show because of comparison here:
https://core.trac.wordpress.org/browser/trunk/src/wp-admin/user-edit.php#L420

Looks like this has been an issue since about 3.1, maybe older.

relevant commits: [26115], [16767]

[SergeyBiryukov]

Introduced in [16767]. A few similar instances were fixed in [26115], but this one was missed for some reason.

[SergeyBiryukov]

In 45698:

Users: Use case-insensitive email address comparison when checking whether "Grant this user super admin privileges" checkbox should be displayed on Edit User screen.

Props trepmal.
Fixes #47119.