Simplify the document for etcd auto-compaction

[ahrtr]

trafficstars

etcd v3.3 has already been out of support, so we don't need to talk too much about the old behaviour for 3.3 in https://etcd.io/docs/v3.5/op-guide/maintenance/#auto-compaction. It has no any benefit other than confusing the end users. Instead, we just need to summarize the latest behaviour.

If users still need to look into the historical behavior, they just need to look at 3.4 or older version's document.