chore: switch geant4 to qt6

This PR switches geant4 visualization from Qt5 to Qt6, taking the installation of Qt from 270 MB down to 70 MB.

spack.sh

@@ -18,6 +18,7 @@ f73d7d2dce226857cbc774e942454bad2992969e
 cbab451c1a342523ed75e9be1098615a597a9b59
 b894acf1fca8eb5cc52d2267b0c4c187065998c0
 8b45fa089e24c6ab7de2eaf614977369e69daa54
+2d8ca8af6932dfd50204d1e4f6fe587dec7beef5
 ---
 ## Optional hash table with comma-separated file list
 read -r -d '' SPACK_CHERRYPICKS_FILES <<- \
@@ -35,3 +36,4 @@ read -r -d '' SPACK_CHERRYPICKS_FILES <<- \
 ## cbab451c1a342523ed75e9be1098615a597a9b59: dd4hep: Add version 1.29
 ## b894acf1fca8eb5cc52d2267b0c4c187065998c0: geant4: add v11.2.2, incl g4ndl v4.7.1
 ## 8b45fa089e24c6ab7de2eaf614977369e69daa54: geant4: support Qt5 and Qt6
+## 2d8ca8af6932dfd50204d1e4f6fe587dec7beef5: qt-*: add v6.7.1, v6.7.2