Update mbstring http_input and http_output check to accept UTF-8 configuration.

As an additional refinement in parallel to the phpBB 3.3.3-specific https://tracker.phpbb.com/browse/PHPBB3-16656 change, in addition to having eliminated the now-deprecated "pass" as the expected/required configuration for mbstring.http_input and mbstring.http_output, phpBB should additionally allow "UTF-8" to exist for this configuration.

The reason phpBB was checking for "a blank value, or pass" previously, and the reason phpBB is checking for "a blank value" now in phpBB 3.3.3 and later, isn't just "to randomly enforce PHP configuration rules."  The current phpBB 3.3.3 check intends to ensure that the mbstring.http_input and mbstring.http_output will inherit the PHP "default_charset" configuration, because phpBB in addition requires that "default_charset" is explicitly or at least implicitly configured as "UTF-8".

So when an existing customer server configuration, such as the one in https://www.phpbb.com/community/viewtopic.php?f=661&t=2585241, has their mbstring.http_input and/or mbstring.http_output configuration set to "UTF-8", the fact that the current phpBB 3.3.3 ACP test "doesn't accept this" and shows the warning is not helpful to the phpBB user, and is not advising the phpBB user of a situation that "must be corrected in order to have proper phpBB operation."

UTF-8 is what phpBB needs for proper operation; being explicitly configured for UTF-8 just isn't one of the ways the current phpBB 3.3.3 test accounts for.  Such as in the customer case cited from phpBB Community above, the hosting provider won't allow the customer to change this configuration, either.  So they are currently stuck always seeing this warning, when in actuality, phpBB is already receiving the UTF-8 encoded data it required.

Recommend that the previous "pass, or a blank value" test be replaced with "UTF-8, or a blank value" test.  Along with the same "could be an upper-case or lower-case UTF-8" treatment that was implemented for phpBB's test of "default_charset".