docs(gcc): update GCC to version 10.3-2021.07

This toolchain provides multiple cross compilers and is publicly
available on 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: I673e0dce8eb3ca3a004a43158a948431b032e93a
Signed-off-by: Daniel Boulby <daniel.boulby@arm.com>
diff --git a/docs/getting_started/prerequisites.rst b/docs/getting_started/prerequisites.rst
index aa1ae67..f45193a 100644
--- a/docs/getting_started/prerequisites.rst
+++ b/docs/getting_started/prerequisites.rst
@@ -26,7 +26,7 @@
 |TF-A| can be built with any of the following *cross-compiler* toolchains that
 target the Armv7-A or Armv8-A architectures:
 
-- GCC >= 10.2-2020.11 (from the `Arm Developer website`_)
+- GCC >= 10.3-2021.07 (from the `Arm Developer website`_)
 - Clang >= 4.0
 - Arm Compiler >= 6.0