[automated testing] debian testing: compile failure with qt6

[testing at tester.lyx.org]

Making all in autotests
Making all in config
Making all in development
Making all in po
Making all in 3rdparty
Making all in nod
Making all in src
  CHK      lyx_commit_hash.h
Making all in support
Making all in frontends
Making all in qt
  CXX      DialogFactory.o
In file included from GuiPrefs.h:33,
                 from DialogFactory.cpp:47:
ui_PrefColorsUi.h: In member function 'void Ui_PrefColorsUi::setupUi(QWidget*)':
ui_PrefColorsUi.h:105:44: error: 'EditUndo' is not a member of 'QIcon'
  105 |         QIcon icon(QIcon::fromTheme(QIcon::EditUndo));
      |                                            ^~~~~~~~
ui_PrefColorsUi.h:113:45: error: 'EditRedo' is not a member of 'QIcon'
  113 |         QIcon icon1(QIcon::fromTheme(QIcon::EditRedo));
      |                                             ^~~~~~~~
ui_PrefColorsUi.h:184:45: error: 'GoPrevious' is not a member of 'QIcon'
  184 |         QIcon icon2(QIcon::fromTheme(QIcon::GoPrevious));
      |                                             ^~~~~~~~~~
ui_PrefColorsUi.h:197:45: error: 'GoNext' is not a member of 'QIcon'
  197 |         QIcon icon3(QIcon::fromTheme(QIcon::GoNext));
      |                                             ^~~~~~
make[6]: *** [Makefile:1023: DialogFactory.o] Error 1
make[5]: *** [Makefile:857: all] Error 2
make[4]: *** [Makefile:804: all-recursive] Error 1
make[3]: *** [Makefile:2625: all-recursive] Error 1
make[2]: *** [Makefile:1701: all] Error 2
make[1]: *** [Makefile:586: all-recursive] Error 1
make: *** [Makefile:481: all] Error 2