Autobuild: Mac: Build with Qt 6.2.2 + Xcode 12.4

Qt 6.2.2 is the latest supported version at the time of writing.
Xcode 12.4 is the latest supported version for macos-10.15.

.github/workflows/autobuild.yml

@@ -111,7 +111,7 @@ jobs:
            - config_name:           MacOS (codeQL)
              target_os:             macos
              building_on_os:        macos-10.15
-             cmd1_prebuild:         "./autobuild/mac/codeQL/autobuild_mac_1_prepare.sh 5.15.2"
+             cmd1_prebuild:         "./autobuild/mac/codeQL/autobuild_mac_1_prepare.sh 6.2.2"
              cmd2_build:            "./autobuild/mac/codeQL/autobuild_mac_2_build.sh"
              cmd3_postbuild:        false
              uses_codeql:           true
@@ -120,11 +120,15 @@ jobs:
              target_os:             macos
              # Stay on 10.15 as long as we use dmgbuild which does not work with 11's hdiutil (?)
              building_on_os:        macos-10.15
-             cmd1_prebuild:         "./autobuild/mac/artifacts/autobuild_mac_1_prepare.sh 5.15.2"
+             cmd1_prebuild:         "./autobuild/mac/artifacts/autobuild_mac_1_prepare.sh 6.2.2"
              cmd2_build:            "./autobuild/mac/artifacts/autobuild_mac_2_build.sh sign_if_possible"
              cmd3_postbuild:        "./autobuild/mac/artifacts/autobuild_mac_3_copy_files.sh"
              uses_codeql:           false
-             xcode_version:         11.7
+             # Fix Xcode version to avoid surprises. Can be bumped after testing and checking deployment
+             # target compatibility at https://developer.apple.com/support/xcode/ and
+             # https://doc.qt.io/qt-6/macos.html
+             # Use 12.4 for now which is the last version which works on 10.15.
+             xcode_version:         12.4
 
            - config_name:           MacOS Legacy (artifacts)
              target_os:             macos