When upgrading to `utf8mb4`, `TEXT` fields will be upgraded to `MEDIUMTEXT` (and likewise for all other `*TEXT` and `*BLOB` fields). This is to allow for the additional space requirements of `utf8mb4`.
On the subsequent upgrade, after the `utf8mb4` upgrade, `dbDelta()` would try and downgrade the fields to their original size again. At best, this it a waste of time, at worst, this could truncate any data larger than the original size. There's no harm in leaving them at their original size, so let's do that.
Fixes#36748.
git-svn-id: https://develop.svn.wordpress.org/trunk@37525 602fd350-edb4-49c9-b593-d223f7449a82