Upgrade the version of ChromiumOS used in KernelCI to R124

[a-wai]

KernelCI currently uses ChromiumOS images based on R120, released in January. however, we hit Tast bugs with this release and should consider upgrading to the next stable version.

R124 is currrently in beta stage and will be released on April 30th and is therefore our next target.

• [x] Upgrade all overlays to R124
• [x] Refresh / revisit patches carried in R120
• [ ] Move Mediatek kernels from our 6.6 fork to chromeos-kernel 6.6 (verify the 3 patches from our fork are present) Switched back to ChromeOS downstream kernel as there is little interest in having a mainline kernel shipping by default (we test those already in KernelCI); downside is the downstream kernel doesn't work, likely due to using proprietary Mali display drivers
• [x] Build, flash and run some tests on various boards to quickly identify problems
• [x] Do "official" builds for boards
• [x] Flash the official image on boards and run Tast

[a-wai]

Results analysis is still in progress, but the R124 upgrade in itself is complete, so closing this.