osd/osd_types: Remove Snapset::snaps compat (1/2)

osd/osd_types: Remove Snapset::snaps compat (1/2)

In continuation to #28330
From Q release, we should no longer support pre O compatibilty.

Update SnapSet::encode/decode to use an empty instance instead to not
break backward compatibility.

See :
```
struct SnapSet {
  ...
  // NOTE: this is for pre-octopus compatibility only! remove in Q release
  std::vector<snapid_t> snaps;    // descending
```

Signed-off-by: Matan Breizman <mbreizma@redhat.com>