[ci] Upgrade to GCC14

[salkinium]

Let's see what breaks…

• avr-gcc 14 now delegates __atomic_test_and_set to the library: https://gcc.gnu.org/bugzilla/show_bug.cgi?id=118280
• avr-gcc 14 seems to have a size regression and overflows the unittests for the Arduino Uno and Nano. This is not an issue for us, since the unittests still fit in their entirety on the ATMega2560.
• Windows arm-none-eabi-gcc 14 has a different unzip folder structure, it unzips directly into the folder, instead of a subfolder, causing the /bin path not to be correct.