[doc] update docs on clang-format version

CONTRIBUTING.md

@@ -114,7 +114,9 @@ At the same time, this freedom comes with certain responsibilities, which are ro
 
 Before contributing, please make sure to run `clang-format` on your C++ (.h and
 .cpp) files. The codebase is currently formatted with `clang-format` version
-`18.1.8`.
+`18.1.3`. This is the default `clang-format` version when installing via `apt`
+on Ubuntu 24.04, and can easily be installed via `python -mpip install clang-format==18.1.3`
+on other systems.
 We provide a `.clang-format` file at the root of the repository. Most text
 editors and IDEs will have some kind of integration for detecting that file
 and automatically applying `clang-format`. `git-clang-format` can be used to