Allow mod added tables to be mb4 when smf is running in mb4

[albertlast]

Based on the discussion #6405 i checked the existing mb4 logic of smf and found a spot which got not covered atm.

In general when someone plan to convert his smf 2.1 to mb4, the programm logic expect that not only the existing column get changed, also the default setting of the tables needs to be changed to mb4.

[Sesquipedalian]

~Note: This PR targets release-2.1, but it has been assigned to the milestones for SMF 3.0. The PR will need to be updated and resubmitted to target release-3.0. I am leaving it open for now so that it doesn't get forgotten.~

Actually, this PR will become moot once we convert SMF 3.0 to use utf8mb4 by default on MySQL. But this code will likely be for the latter, so I am leaving it open for now to make it easier to find for reference.