macOS: Bump deployment target (minimum supported version) to 10.12

As discussed earlier, we don't want to keep backwards compatibility
for more than two versions in addition to the current macOS version.

Change-Id: I24df6fb4a08e14a9f842d209b8e0a6079c533b65
Reviewed-by: Oswald Buddenhagen <[email protected]>
Reviewed-by: Morten Johan Sørvig <[email protected]>

mkspecs/common/macx.conf

@@ -5,7 +5,7 @@
 QMAKE_PLATFORM         += macos osx macx
 QMAKE_MAC_SDK           = macosx
 
-QMAKE_MACOSX_DEPLOYMENT_TARGET = 10.11
+QMAKE_MACOSX_DEPLOYMENT_TARGET = 10.12
 QMAKE_APPLE_DEVICE_ARCHS = x86_64
 
 device.sdk = macosx