fix: Limit index size

[MissAllSunday]

Fixes #6163

[sbulen]

There are actually 3 indexes that are too small, this only addresses one of them.

And the DB doesn't actually make use of mb4 until we look into this PR: #6409

Until then, it's mb3 data stuffed into mb4-capable tables...

[sbulen]

And 249 is too big, it needs to be 191, for reasons explained over on #6409 also.

[sbulen]

I clearly worry too much... I am also concerned that the string 'utf8' != the string 'utf8mb4'...

There are several places throughout our code - still old 2.0 code that supports ISO-8859-1 - where the logic treats the contents as EITHER utf8 OR ISO-8859-1... I.e., a utf8mb4 DB may actually get treated as a latin1 DB, because it's not 'utf8'. Hence the importance of ripping out all the old "otherwise its ISO-8859-1" logic.

[sbulen]

One last change needed - When we change the DB we also add a corrective step to the upgrader. In this instance, dropping the old index & adding the new one.

An example:
https://github.com/SimpleMachines/SMF2.1/blob/a8f4d6af6465d5990d4df4bd0cb5aed242025a7c/other/upgrade_2-1_mysql.sql#L567

This allows the upgrader to be rerun to fix issues, bring the DB into conformity.