Use clang-format 8

[stertingen]

As clang-format 3.9 is not available on Focal, I would propose to set the recommended version of clang-format to 8 which is available on Xenial, Bionic and Focal.

See also: https://packages.ubuntu.com/search?searchon=names&suite=all&section=all&keywords=clang-format

(Unfortunately, the same .clang-format file leads to different results when used on different versions of clang-format.)