Update auf Nextcloud Hub 18.0.6 - Done

Before the bigger Nextcloud 19 upgrade we update all clouds to 18.0.6.

Todos:

• Wait for the bugfix release 18.0.6 (done)
• Mail App Update to Version 1.4.0 (done)
  
• Talk App Update to Version 8.0.9 (done)
  
• Adjustment of the autoinstallation (done)
  
• New customers get Nextcloud 18.0.4 installed (done)
• Eat your own Dogfood. Update of our own clouds  (done)
• Test the migration of existing customers  (done)
• Migraition of all customers (100% done)