github: update fedora version for clang-format

We use clang-format, but the actual result depends on the version of the tool.
The version to be used, is the one from the github action. You can use
"tools/clang-format-container.sh" to run that version in a container.

Bump the Fedora version (and the used clang-format version) from Fedora
39 (17.0.6-2.fc39) to Fedora 40 (18.1.1-1.fc40). Luckily, our code base
formats the same under both.

.github/workflows/ci.yml

@@ -8,7 +8,7 @@ jobs:
   clang-format:
     runs-on: ubuntu-latest
     container:
-      image: fedora:39
+      image: fedora:40
     steps:
       - name: Install packages
         run: |