Chromium on core24 base

I’m preparing the Chromium transition from core22 base to core24.

Three channels already have it: edge (based on Chromium 128), beta (Chromium 127) and stable/core24 (Chromium 126).

Either can be tried with

snap refresh --channel stable/core24 chromium
snap refresh --beta chromium
snap refresh --edge chromium

If testing it, please let us know of any regression.

As part of the transition, Armhf will no longer be built starting with Chromium 127. This is because the Gnome SDK, on which the build depends, is not built for that architecture on core24.