deprecate validate_email()

[Denis-de-Bernardy]

wp-includes/ms-functions.php contains new functions such as is_email_address_unsafe() and validate_email().

we should at least deprecate validate_email() in favor of is_email().

The is_email_address_unsafe is for preventing signups using email addresses that are banned by a super admin.

(In [12863]) deprecate validate_email(), fixes #11745

added '_deprecated_function' to 'validate_email'

Just need to add the '_deprecated_function' function inside validate_email function so so people can be advised to update.

Committer: Should be 'is_email()'.

[12868]