package/qt-base: add conflict for older gcc (#34420)

var/spack/repos/builtin/packages/qt-base/package.py

@@ -82,6 +82,9 @@ class QtBase(CMakePackage):
         depends_on("libproxy")
         depends_on("openssl")
 
+    # Qt6 requires newer compilers: see https://github.com/spack/spack/issues/34418
+    conflicts("%gcc@:7")
+
     @property
     def archive_files(self):
         """Save both the CMakeCache and the config summary."""