Build: Update GCC default version to 10.3

Signed-off-by: Xinyu Zhang <xinyu.zhang@arm.com>
Change-Id: I4fb087c2be59595b27a6f9bb4d3111c74267e0b8
diff --git a/tf-m-build-config.yaml b/tf-m-build-config.yaml
index b0e5828..4c584dd 100644
--- a/tf-m-build-config.yaml
+++ b/tf-m-build-config.yaml
@@ -36,7 +36,7 @@
           description: |
             Specifies the compiler toolchain.
             For more information see <a href="https://git.trustedfirmware.org/TF-M/trusted-firmware-m.git/plain/docs/getting_started/tfm_build_instruction.rst">here</a>.
-          default: 'GCC_7_3_1'
+          default: 'GCC_10_3'
       - string:
           name: 'CMAKE_BUILD_TYPE'
           description: |