Fix php 8 compatibility warnings, #6698

See merge request !1895

Misc German translation updates

See merge request ispconfig/ispconfig3!1894

Introduce pasting of email address to select the domain also to mail forwards and aliases

Closes #6694

See merge request ispconfig/ispconfig3!1887

Update supported Debian /Ubuntu versions

See merge request ispconfig/ispconfig3!1893

Add final newline to the autorized_keys file, #6697

See merge request ispconfig/ispconfig3!1892

Add comment to identity master.cf sections

Closes #6696

See merge request ispconfig/ispconfig3!1888

Fix access of non existing variable backup.inc.php

See merge request ispconfig/ispconfig3!1884

Fix access of non existing variable backup.inc.php

See merge request ztk.me/ispconfig3!2

in file server/lib/classes/backup.inc.php
The restoreBackupWebFiles method gets passed a $filename
In one branche it is getting concatenated into $full_filename
This fixes the variable name from $full_filename to $filename on line 451
Line 451 is outside the branch described above

Update the serial after the actual insert, #6690

Closes #6690

See merge request ispconfig/ispconfig3!1883