[cleanup] Remove migrations for 3.21 and less
If someone still uses a very old version of CubicWeb, he will not go in one jump to the last version, so we can get rid of very old migrations in CubicWeb 3.31.
If someone still uses a very old version of CubicWeb, he will not go in one jump to the last version, so we can get rid of very old migrations in CubicWeb 3.31.