Bump necessary magic numbers for long-read index types

We need to make sure that long-read Giraffe can't mistake indexes from the current release of vg for indexes it can use, and visa versa. We might need to e.g. change the magic numbers on minimizer indexes to reflect that they now can have zipcodes in them, or make sure that Distance Index v2 doesn't try to accidentally read a v1 distance index file.