Support `ubuntu-latest`/ `ubuntu-24.04` in CI

ubuntu-latest has been updated to 24. This breaks a couple of things:

- gcc 11 is not shipped anymore, but we can run gcc 12 and gcc 14
- either our LLVM install action or the way we configure clang in CMake gets weird: when compiling gRPC, it runs `protoc` (also compiled from source), but then complains that some abi file was not found