`MetaDeleteMarker::decode_from` returned an error on any field key it didn't
recognize, unlike `MetaObject::decode_from` / `MetaObjectV1::decode_from`, which
skip unknown fields. That breaks forward compatibility: a delete marker written
by a newer version with an extra key fails to decode on an older binary.
Skip the unknown field's value (`skip_msgp_value`) and continue, matching the
object decoders. Adds a regression test.
Refs backlog#799 (B17), tracked in rustfs/backlog#863.