Add $medium-up sizes for h5 and h6

This code was probably forgotten in https://github.com/zurb/foundation/pull/5170. `h5` and `h6`'s mobile sizes can now be changed by `$h5-font-reduction` and `$h6-font-reduction`, so they need to be set again within `@media #{$medium-up} { ... }`.

This was unnecessary prior to https://github.com/zurb/foundation/pull/5170 because `h5` and `h6` always had the same font sizes.