Use clang-format 11

[utokusa]

After ubuntu 21 becomes available on github actions, maybe I can move to clang-format 13.

[utokusa]

For mac I installed clang-format 11 by following:

$ brew install clang-format@11
$ sudo mkdir -p /usr/local/bin
$ sudo ln -s $(which clang-format-11) /usr/local/bin/clang-format

https://stackoverflow.com/questions/68057091/how-to-make-a-system-wise-command-alias-on-macos