Raw logs are stored in a versioned binary format and must be update with
atopconvert(1) upon atop version updates.
Failure to do so results in atop.service startup failure as I found out
the hard way after the "atop: 2.6.0 -> 2.7.1"[0] bump:
```
May 31 01:49:25 <hostname> sh[2269709]: existing file /var/log/atop/atop_20220531 has incompatible header
May 31 01:49:25 <hostname> sh[2269709]: (created by version 2.6 - current version 2.7)
May 31 01:49:25 <hostname> systemd[1]: atop.service: Main process exited, code=exited, status=7/NOTRUNNING
```
Convert logs in `ExecStartPre` and replace them iff updated.
This is to avoid changing original modification times upon every service
start and thus work against atop's log rotation (see existing
`ExecStartPre`).
0: https://github.com/NixOS/nixpkgs/pull/175180#issuecomment-1141546487
