Much easier to avoid sstable collisions. Makes it possible to scrub
multiple sstables, with multiple calls to scylla-sstable, reusing the
same output directory. Previously, each new call to scylla-sstable
scrub, would start from generation 0, guaranteeing collision.
Remove the unit test for generation clash -- with UUID generations, this
is no longer possible to reproduce in practice.
Refs: #21387Closesscylladb/scylladb#23990