ci: target macOS 11.0 in CI

The Qt6 binaries target 11.0, so there's no point in testing 10.15
directly here.

Note that 10.15 is required to allow Qt access to use `std::filesystem`
from the SDKs as well.

.gitlab/ci/configure_macos_x86_64_kits_python_qt.cmake

-# Ensure that we're targeting 10.13.
-set(CMAKE_OSX_DEPLOYMENT_TARGET "10.13" CACHE STRING "")
+# Ensure that we're targeting 11.0.
+set(CMAKE_OSX_DEPLOYMENT_TARGET "11.0" CACHE STRING "")
 
 include("${CMAKE_CURRENT_LIST_DIR}/configure_macos.cmake")