Update system requirements in documentation

Signed-off-by: Felix Weilbach <felix.weilbach@nextcloud.com>

doc/installing.rst

@@ -32,13 +32,8 @@ System Requirements
 ----------------------------------
 
 - Windows 10+
-- macOS 10.12+ (**64-bit only**)
-- macOS 10.10 & 10.11 (**64-bit and up to client legacy version 2.6.5 only**)
-- CentOS 6 & 7 (64-bit only)
-- Debian 8.0 & 9.0
-- Fedora 25 & 26 & 27
-- Ubuntu 16.04 & 17.04 & 17.10
-- openSUSE Leap 42.2 & 42.3
+- macOS 10.12+ (64-bit only)
+- Linux
 - FreeBSD
 
 .. note::