According to https://volkerkrause.eu/2023/12/16/kf6-android-porting-status.html, removing this dependency would save us about 4Mb on Android.

We need to check on which cases we can use https://doc.qt.io/qt-6/qml-qtquick-effects-multieffect.html and for which it is not possible.
This MR provides some examples on how to replace them: https://invent.kde.org/plasma/plasma-welcome/-/merge_requests/146 (comments contain a solution for Glow) and this blog may be useful: https://ekkesapps.wordpress.com/qt-6-qmake/from-qt-graphical-effects-to-multi-effect/

Usage: https://invent.kde.org/search?search=Qt5Compat&nav_source=navbar&project_id=2552&group_id=1564&search_code=true&repository_ref=7b1d22fdff076469e9c30e2db1ae9235023075e3

DropShadow:

• algebra_by

(fixed in https://invent.kde.org/education/gcompris/-/commit/dc6a32226b9cdc157a32140854d56c895938df57 )

• balancebox, balancebox

(fixed in https://invent.kde.org/education/gcompris/-/commit/41bbb2becf685fc1d6e8568c42adf19428ecd559 )

• click_on_letter

(fixed in https://invent.kde.org/education/gcompris/-/commit/0d9865db9b9fe224666d132660bb0cd429831e94 )

• fifteen

(fixed in https://invent.kde.org/education/gcompris/-/commit/d929a2742e6baaee10967c08607d99ef4932573f )

• gletters, gletters

(fixed in https://invent.kde.org/education/gcompris/-/commit/a120bb4910a1f0cdff74f8154ef291512830102e )

• redraw, redraw

(fixed in https://invent.kde.org/education/gcompris/-/commit/60fffcc662587bb5bfb546a8ff282d818cd21471 )

• scalesboard

(fixed in https://invent.kde.org/education/gcompris/-/commit/57f4b1cb9cea10f5e1ccfaf181230f76b297ef34 )

• core

(fixed in https://invent.kde.org/education/gcompris/-/commit/07efc6a6c9b18b0bcc490634c7b9790a3cd057cd )

Glow:

• babymatch

(fixed in https://invent.kde.org/education/gcompris/-/commit/4a38fb89ef80656fd16a34af7c0a56a09846a357 )

OpacityMask:

• categorization

(fixed in https://invent.kde.org/education/gcompris/-/commit/e8cfbd0e04992cd132a321266e7971dbc58c03ac )

• crane

(fixed in https://invent.kde.org/education/gcompris/-/commit/3741d2f28a3d7daff64d0f2e5e1fd70b8cdb2f35 )

• lang

(fixed in https://invent.kde.org/education/gcompris/-/commit/01c6bfa341273fce0ec7a39b329391396357ca8a )

• menu

(fixed in https://invent.kde.org/education/gcompris/-/commit/120f6c7a2283e199e613284e1fd8d4797869df96 )

ThresholdMask:

• hangman

(fixed in https://invent.kde.org/education/gcompris/-/commit/ad1423a72c6867f7c2bff3d5efe9e2753bff8687 )

ColorOverlay:

• hexagon

(fixed in https://invent.kde.org/education/gcompris/-/commit/0b87c2a252b619685568bfd7c0e28f0c13fca07c with a better solution which works also with software-render)

Note: when totally removed, also remove from CMakeLists.txt, HACKING and HACKING.
(done in https://invent.kde.org/education/gcompris/-/commit/edddc3deeee93cf116c2c855303cefe204b25291 )