docs(build): update GCC to 11.3.Rel1 version

This toolchain provides multiple cross compilers and is publicly
available on https://developer.arm.com/



We build TF-A in CI using:
AArch32 bare-metal target (arm-none-eabi)
AArch64 ELF bare-metal target (aarch64-none-elf)

Change-Id: I94e13f6c1ebe3a4a58ca6c79c1605bd300b372d3
Signed-off-by: Jayanth Dodderi Chidanand <jayanthdodderi.chidanand@arm.com>