Bump macOS support from El Capitan to Sierra

GitHub Desktop recently shipped a version of Git LFS which has issues running on macOS 10.11. Given that 10.11 is unsupported from Apple it's about time we bumped this.

Fixes desktop/desktop#12921

cc @github/desktop

Author: niik

data/variables/desktop.yml

@@ -1,5 +1,5 @@
 # Supported platforms
 
-mac-osx-versions: macOS 10.10 or later
+mac-osx-versions: macOS 10.12 or later
 
 windows-versions: Windows 7 64-bit or later