Change fixed clang-format version to 15

* We commit statically linked x64 binaries for clang-format-15 for windows and
  linux to make the transition easier for anyone running a linux distro that
  doesn't yet support llvm 15, or in future for distros that drop support for
  it.

.github/workflows/ci.yml

@@ -42,17 +42,7 @@ jobs:
     - uses: actions/checkout@v3
       with:
         fetch-depth: 5
-    - name: Add LLVM apt repository
-      run: |
-        wget -O - http://apt.llvm.org/llvm-snapshot.gpg.key | sudo apt-key add -
-        sudo add-apt-repository -y 'deb http://apt.llvm.org/xenial/ llvm-toolchain-xenial-3.8 main'
-        sudo apt-get update
-        curl -LO http://archive.ubuntu.com/ubuntu/pool/main/libf/libffi/libffi6_3.2.1-8_amd64.deb
-        sudo dpkg -i libffi6_3.2.1-8_amd64.deb
-    - name: Install clang-format-3.8
-      run: |
-        sudo apt-get install -y clang-format-3.8
-    - name: Run clang-format-3.8
+    - name: Run clang-format
       run: |
         . ./util/clang_format_all.sh
         git clean -f