gitlab-ci: use Xcode 12.3

This opens the door to supporting macOS 11 and aarch64 binaries.

.gitlab/os-macos.yml

@@ -7,7 +7,7 @@
         GIT_CLONE_PATH: "$CI_BUILDS_DIR/paraview-ci-ext/$CI_CONCURRENT_ID"
         # TODO: Factor this out so that each job selects the Xcode version to
         # use so that different versions can be tested in a single pipeline.
-        DEVELOPER_DIR: "/Applications/Xcode-11.5.app/Contents/Developer"
+        DEVELOPER_DIR: "/Applications/Xcode-12.3.app/Contents/Developer"
 
 ### Build and test
 
@@ -36,7 +36,7 @@
         - paraview
         - macos
         - shell
-        - xcode-11.5
+        - xcode-12.3
         - nonconcurrent
 
 ## macOS-specific scripts