[ci] Update clang-format to v18

This breaks the previous convention of staying behind one version
after the last major release, because:
  - clang-format's base preset configs change over time (so the same
    config using "BasedOnStyle" may have a slightly different style
    with new clang-format versions),
  - it's a hassle to have one version locally and another in the CI,
  - no-one else is really contributing much at this point.

.github/workflows/check-and-build.yml

@@ -13,7 +13,7 @@ jobs:
 
       - name: Install clang-format
         run: |-
-          clang_major_version=17
+          clang_major_version=18
           set -x -u -o pipefail
           source /etc/os-release
           wget -O - https://apt.llvm.org/llvm-snapshot.gpg.key | sudo apt-key add -