update cron to persistence mixin

[h00die]

Updates cron_persistence to the new persistence mixin. Part of #20374

Verification

• Start msfconsole
• exploit the box somehow (ssh_login for instance)
• use exploit/multi/persistence/cron
• set SESSION <id>
• exploit
• Verify persistence is created, and you get a new session if apt is run
• Verify cleanup works
• Document is updated and correct