Migration to Nextcloud 16 - Released

• Talk app update to version 6.0 (done)
• Mail app update to version 0.15 (done)
• Calendar app Update Version 1.7.0 (done)
• End2End Encryption needs a release (Github)
• Tasks app updated to version 0.10 (done)
• Adjustment of the autoinstallation (done)
• New customers get Nextcloud 16 installed (done)
• Update all customers clouds to the last version of Nextcloud 15 (done)
• Test the migration of existing customers  (done)
  
• Eat your own Dogfood. Update of our own clouds (done)
  
• Migraition of all customers (done)

(The todos are sorted by priority.)