bump clang-format to clang-15

clang 11 was released in 2007, and the last variant of Ubuntu with the
clang-format-11 package is 22.04.

This bumps the clang-format used for the formatting check to clang 15,
available with 24.04.

.github/workflows/syntax-checks.yaml

@@ -6,7 +6,7 @@ on:
 jobs:
   # This job takes approximately 1 minute
   check-clang-format:
-    runs-on: ubuntu-20.04
+    runs-on: ubuntu-24.04
     steps:
       - uses: actions/checkout@v4
         with:
@@ -19,7 +19,7 @@ jobs:
           DEBIAN_FRONTEND: noninteractive
         run: |
           sudo apt-get update
-          sudo apt-get install --no-install-recommends -yq clang-format-11
+          sudo apt-get install --no-install-recommends -yq clang-format-15
       - name: Check updated lines of code match clang-format-11 style
         env:
           BASE_BRANCH: ${{ github.base_ref }}

CODING_STANDARD.md

@@ -341,13 +341,12 @@ To avoid waiting until you've made a PR to find formatting issues, you can
 install clang-format locally and run it against your code as you are working.
 
 Different versions of clang-format have slightly different behaviors. CBMC uses
-clang-format-11 as it is available the repositories for Ubuntu 20.04 and
-Homebrew.
-To install on a Unix-like system, try installing using the system package
-manager:
+clang-format-15, available in the standard Ubuntu 24.04 and Homebrew
+repositories. To install on a Unix-like system, try installing using the
+system package manager:
 ```
-apt-get install clang-format-11  # Run this on Ubuntu, Debian etc.
-brew install clang-format@11     # Run this on a Mac with Homebrew installed
+apt-get install clang-format-15  # Run this on Ubuntu, Debian etc.
+brew install clang-format        # Run this on a Mac with Homebrew installed
 ```
 
 If your platform doesn't have a package for clang-format, you can download a