burp-client: Switch from cron to systemd timer

systemd timer units are supported on all relevant OS versions now.
There is no longer any reason to use cron.

roles/burp-client/files/burp-backup.service

@@ -0,0 +1,27 @@
+# vim: set ft=systemd :
+[Unit]
+Description=BURP client
+After=network-online.target
+Wants=network-online.target
+
+[Service]
+Type=exec
+ExecStart=/usr/sbin/burp -a t -Q
+SuccessExitStatus=3
+CapabilityBoundingSet=CAP_BLOCK_SUSPEND CAP_DAC_OVERRIDE CAP_DAC_READ_SEARCH
+CapabilityBoundingSet=CAP_FOWNER CAP_LEASE CAP_SETGID CAP_SETUID
+NoNewPrivileges=yes
+PrivateDevices=yes
+PrivateTmp=yes
+ProcSubset=pid
+ProtectClock=yes
+ProtectControlGroups=yes
+ProtectHostname=yes
+ProtectKernelLogs=yes
+ProtectKernelModules=yes
+ProtectKernelTunables=yes
+ProtectProc=noaccess
+ProtectSystem=full
+SystemCallArchitectures=native
+SystemCallFilter=@system-service @privileged @mount
+SystemCallFilter=~@clock @debug @module @reboot @swap