Build: Update iOS Runner and SDK

This updates the iOS build to a newer SDK which is not supported by Qt5.
Due to ongoing work on Qt6 support, this is a hotfix only with the goal
to have a fully working Qt6 iOS build in future.

.github/workflows/autobuild.yml

@@ -251,13 +251,13 @@ jobs:
 
           - config_name:            iOS (artifacts)
             target_os:              ios
-            building_on_os:         macos-10.15
+            building_on_os:         macos-12
             base_command:           QT_VERSION=5.15.2 ./.github/autobuild/ios.sh
             # Build failed with CodeQL enabled when last tested 03/2022 (#2490).
             # There are no hints that iOS is supposed to be supported by CodeQL.
             # Therefore, disable it:
             run_codeql:             false
-            xcode_version:          12.1.1
+            xcode_version:          13.4.1
 
           - config_name:            Windows (artifact+codeQL)
             target_os:              windows