ci: Build image with LLVM tools 21 (#3049)

docker/ci/Dockerfile

@@ -61,7 +61,7 @@ RUN pip install -q --no-cache-dir \
         pre-commit
 
 # Install LLVM tools
-ARG LLVM_TOOLS_VERSION=20
+ARG LLVM_TOOLS_VERSION=21
 
 RUN echo "deb http://apt.llvm.org/focal/ llvm-toolchain-focal-${LLVM_TOOLS_VERSION} main" >> /etc/apt/sources.list \
     && wget --progress=dot:giga -O - https://apt.llvm.org/llvm-snapshot.gpg.key | apt-key add -

docker/ci/README.md

@@ -14,6 +14,7 @@ The image is based on Ubuntu 20.04 and contains:
 - GDB 17.1
 - gh 2.83.2
 - git-cliff 2.11.0
+- LLVM Tools 21
 - mold 2.40.4
 - Ninja 1.13.2
 - Python 3.8