[Github] Bump LLVM Version in CI Container to 21.1.3

Just to keep things up to date.

Author: boomanaiden154

.github/workflows/containers/github-action-ci/Dockerfile

@@ -2,7 +2,7 @@ FROM docker.io/library/ubuntu:24.04 AS base
 ENV LLVM_SYSROOT=/opt/llvm
 
 FROM base AS stage1-toolchain
-ENV LLVM_VERSION=21.1.1
+ENV LLVM_VERSION=21.1.3
 
 RUN apt-get update && \
     apt-get install -y \