Bump DEPLOYMENT_TARGET to 10.15.

Needed for some filesystem features used in imageio.

CMakeLists.txt

@@ -117,7 +117,7 @@ if(APPLE)
     # When changing the target you must also edit the triplet files in
     # vcpkg-triplets to reflect the new target.
     if(APPLE_MAC_OS)
-        set(CMAKE_OSX_DEPLOYMENT_TARGET "10.13" CACHE STRING "macOS Deployment Target")
+        set(CMAKE_OSX_DEPLOYMENT_TARGET "10.15" CACHE STRING "macOS Deployment Target")
     elseif(CMAKE_SYSTEM_NAME STREQUAL "iOS" OR CMAKE_SYSTEM_NAME STREQUAL "tvOS")
         set(CMAKE_OSX_DEPLOYMENT_TARGET "12.0" CACHE STRING "iOS/tvOS Deployment Target")
         set(CMAKE_XCODE_ATTRIBUTE_ONLY_ACTIVE_ARCH NO)