The reported starting height of a peer in the VERSION message is
untrusted, and it doesn't seem to be useful anymore (after #20624),
so deprecating the corresponding "startingheight" field seems
reasonable. After that, it can be removed, along with the
`m_starting_height` field of the Peer / CNodeStats structs, as it is
sufficient to show the reported height only once at connection in the
debug log.
