Update unattended-upgrades.Ubuntu.erb

Warning: Variable access via 'lsbdistcodename' is deprecated. Use '@lsbdistcodename' instead. template[modules/apt/templates/unattended-upgrades.Ubuntu.erb]:2
   (at modules/apt/templates/unattended-upgrades.Ubuntu.erb:2:in `result')
Warning: Variable access via 'lsbdistcodename' is deprecated. Use '@lsbdistcodename' instead. template[modules/apt/templates/unattended-upgrades.Ubuntu.erb]:3
   (at modules/apt/templates/unattended-upgrades.Ubuntu.erb:3:in `result')
Warning: Variable access via 'mail' is deprecated. Use '@mail' instead. template[modules/apt/templates/unattended-upgrades.Ubuntu.erb]:18
   (at modules/apt/templates/unattended-upgrades.Ubuntu.erb:18:in `result')
Warning: Variable access via 'mail' is deprecated. Use '@mail' instead. template[modules/apt/templates/unattended-upgrades.Ubuntu.erb]:19
   (at modules/apt/templates/unattended-upgrades.Ubuntu.erb:19:in `result')