Fix 3572: macos-15 runner does not support older than Qt 6.9.1

Author: pljones

.github/workflows/autobuild.yml

@@ -250,7 +250,7 @@ jobs:
           - config_name:            iOS (artifacts)
             target_os:              ios
             building_on_os:         macos-15
-            base_command:           QT_VERSION=6.7.3 ./.github/autobuild/ios.sh
+            base_command:           QT_VERSION=6.9.1 ./.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: