[BP 5.5.3] - AppendPrepend makes expire 0

Backport of MB-31070 to 5.5.3

repro:

1. save document with expires
2. append something
3. expires is now 0

one way to see expires:
http://localhost:8091/pools/default/buckets/BUCKET1/docs/KEY1

{

    "meta": {

        "id": "KEY1",

        "rev": "82403-154eb8606b8900000000000000000000",

        "att_reason": "invalid_json",

        "expiration": 0,

        "flags": 0

    },

    "base64": ...

}

we believe, the root of the problem is in this line:
https://github.com/couchbase/kv_engine/blob/0185d9fe79d5aa705ed113184f9436d6f7215325/daemon/protocol/mcbp/appendprepend_context.cc#L187

It just needs to be "oldItemInfo.exptime"