* Support "prepare-upgrade" through "krillup" binary.
* Package the new `krillup` binary and man page into a separate DEB/RPM.
* Also switches to building in a Rocky Linux container due to repeated CentOS 8 vault repo timeouts.
* Mention correct config file location on CLI help.
* Remove incorrect reference to non-existent help subcommand for krillup.
* Add locking to upgrade process.
Co-authored-by: Ximon Eighteen <3304436+ximon18@users.noreply.github.com>