docs(build): update GCC to 12.3.Rel1 version

Updating toolchain to the latest production release version
12.3.Rel1 publicly available on:
https://developer.arm.com/downloads/-/arm-gnu-toolchain-downloads

We build TF-A in CI using x86_64 Linux hosted cross toolchains:
---------------------------------------------------------------
* AArch32 bare-metal target (arm-none-eabi)
* AArch64 bare-metal target (aarch64-none-elf)

Change-Id: Ifcabb7fb9d8e13b87e164c3c1be8c8d32c31b49a
Signed-off-by: Jayanth Dodderi Chidanand <jayanthdodderi.chidanand@arm.com>

docs/getting_started/prerequisites.rst

@@ -26,7 +26,7 @@ Toolchain
 |TF-A| can be built with any of the following *cross-compiler* toolchains that
 target the Armv7-A or Armv8-A architectures:
 
-- GCC >= 12.2.Rel1 (from the `Arm Developer website`_)
+- TF-A has been tested with version 12.3.Rel1 (gcc 12.3) from the `Arm Developer website`_
 
    You will need the targets ``arm-none-eabi`` and ``aarch64-none-elf`` for
    AArch32 and AArch64 builds respectively.