Update Multi-GCC CI?

https://developer.arm.com/downloads/-/arm-gnu-toolchain-downloads now has GCC 14.3.Rel1, so I guess it might be worth adding that to https://github.com/raspberrypi/pico-sdk/blob/master/.github/workflows/multi-gcc.yml ?
But of course that also relies on that compiler being installed on our self-hosted runner too, so ping @liamfraser 🙂 