krita: update to 6.0.4, build with Qt6

[omemoji]

Switches krita from Qt5 to Qt6/KF6, as suggested in #61486. The two dependent packages move with it:

• seexpr-krita 6.0.0.0: Qt6-only, and required by Krita 6 for the SeExpr generator layer (it needs >= 6.0.0.0). Upstream only publishes it as a GitLab tag, so the distfile is the invent.kde.org archive. Krita's own Qt6 dependency set (krita-deps-management, transition.now/qt6) uses the same tag.
• gmic-krita 3.7.2.2: built against Qt6/KF6. The source is now vanyossi/gmic, the tarball Krita itself pins in 3rdparty_plugins/ext_gmic (checksum matches). This addresses the "gmic-krita is not ready" concern raised in krita: update to 6.0.1 #61486.

Other notes:

• post_patch relaxes the SIP ABI pin in SIPMacros.cmake (13.0 -> 13). Without it, the PyKrita bindings fail to build against PyQt6 6.11. The pin is still there in 6.0.4.
• krita uses Qt private API (Qt6GuiPrivate, Qt6WaylandClientPrivate), so it is pinned to qt6-core>=6.11.2 and needs a rebuild on every Qt6 bump.

This needs a policy decision. Upstream still gates Qt6 builds behind -DALLOW_UNSTABLE=QT6:

Building Krita with Qt6 is not yet production-ready. Please don't package this and ship it as a stable version of the software.

https://invent.kde.org/graphics/krita/-/blob/v6.0.4/CMakeLists.txt#L374-381

The same 6.0.4 tarball still builds as Krita 5.3.4 against Qt5, so keeping Qt5 remains an option. I'd like the maintainers' call on whether Void should ship the Qt6 build. cc @Johnnynator @sgn @classabbyamp

Testing the changes

• I tested the changes in this PR: YES
  • Native Wayland startup (niri), GPU canvas acceleration, correct HiDPI scaling
  • G'MIC filters dialog opens and applies filters
  • SeExpr fill layer works
  • seexpr-krita test suite passes (-Q)

Local build testing

• I built this PR locally for my native architecture, (aarch64-glibc)
• Native aarch64 builds of krita need quazip: fix leaked cross sysroot path in cmake exports #62722 (a leaked cross sysroot path in quazip's CMake exports). CI and the official builders cross-compile aarch64, so they are not affected.

[classabbyamp]

if upstream says it's not ready, we probably shouldn't do it

[Johnnynator]

I'm with abby, we should stay with qt5 for now

[omemoji]

@classabbyamp
OK, I'll close this PR when the all CI passes and make it as a new PR again when krita with qt6 turnes stable.