Upgrade 1 arm64 and 1 x86 chromium staging bots to macOS 15.5+

Upgrade the following devices to macOS 15.5+
- [x] [build796-m9](https://chromium-swarm.appspot.com/bot?id=build796-m9)
- [x] [mac-685-h526](https://chromium-swarm.appspot.com/bot?id=mac-685-h526)

Previous example request: b/326078082