Move to snapcraft9/core26 (with Chisel)

Environment/Versions

• GIMP version number: 3.2+
• Package: Snap
• Operating System: Linux

Description of the bug

This is a reminder to myself updating the following parts of build/linux/snap/snapcraft.yaml when core26 snap is released:

• base
• plugs:npu-libs:content (remember to check if there is a new version based on core26)
• apps:gimp:environment:GS_LIB (also check if it is now unversioned)
• parts:buildenv:build-environment /snap/gnome-46-2404-sdk/current/ references should be removed (if snapcraft 13 is being used)
• parts:*:stage-packages should be likely updated and using Chisel slices: https://github.com/canonical/chisel
• The gimp-web-devel documentation should be likely updated mentioning chisel(?): https://testing.developer.gimp.org/core/maintainer/accounts/snapcraft/
• See what needs to be changed on python after https://forum.snapcraft.io/t/core26-bundled-python-changes/50178