Changes expired-authz-purger to use `VersionString`.
Changes expired-authz-purger to use `VersionString`.

Other boulder cmd's have a `const clientName` that is used with
`cmd.VersionString()` to log a bunch of friendly version information.

This commit updates the `expired-authz-purger` to follow the same
paradigm instead of calling `cmd.Version()`, a less useful version
of `cmd.VersionString()`.

ZCloud-Firstserver/boulder,ZCloud-Firstserver/boulder,bmw/boulder,ibukanov/boulder,jfrazelle/boulder,lucasallan/boulder,jsha/boulder,jfrazelle/boulder,jcjones/boulder,lucasallan/boulder,bmw/boulder,letsencrypt/boulder,ZCloud-Firstserver/boulder,jfrazelle/boulder,jcjones/boulder,jsha/boulder,ibukanov/boulder,sophie-h/boulder,jfrazelle/boulder,ZCloud-Firstserver/boulder,letsencrypt/boulder,bmw/boulder,jsha/boulder,letsencrypt/boulder,lucasallan/boulder,jcjones/boulder,sophie-h/boulder,jcjones/boulder,sophie-h/boulder,ibukanov/boulder