Change default to charon-systemd instead of strongswan-starter

This MR makes strongswan runtime depends on charon-systemd instead of strongswan-starter from now on. This will enforce users to use the new systemd unit service file instead of the legacy one (check this comment from one of the upstream maintainers) in new installations. Nevertheless, users will still be able to use strongswan-starter if they want. A summary of the proposed changes:

• Rename strongswan-starter to strongswan-starter-legacy. Keep the strongswan-starter as a transitional package.
• Make strongswan runtime depend on charon-systemd

FWIW the package builds fine and autopkgtest is still happy.