CI: Reduce the min GCC version used to match what MakeCode (PXT) uses.

[microbit-carlos]

MakeCode builds use an older GCC version, so we need to make sure any changes remain compatible.

$ docker run --rm -it pext/yotta:latest arm-none-eabi-gcc --version
arm-none-eabi-gcc (GNU Tools for ARM Embedded Processors) 6.2.1 20161205 (release) [ARM/embedded-6-branch revision 243739]
Copyright (C) 2016 Free Software Foundation, Inc.
This is free software; see the source for copying conditions.  There is NO
warranty; not even for MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.

[github-actions]

Build diff

Base commit: 73329cea9f963fe3129f7f51c2a668dc50c952e5
Action run: https://github.com/lancaster-university/codal-microbit-v2/actions/runs/12871659288

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[finneyj]

Thanks @microbit-carlos - makes sense.