Change update recommendation

Due to the potentially conflicting migrations between 7.3 and 7.4 regarding services, I recommend we change this guide. Until we can backport a 'fix' for the migrations that eliminates the potential problem it is unwise to tell users it's safe to upgrade across 7.3 and 7.4 in one go.

This PR is against master and should also be backported to 7-4-stable.