CI: demand clang-format 12 or 13

- use clang-format12 or 13 or unversioned executable, only
- check it's actually 12 or 13
- set language standard explicitly to C++17, "Cpp11" is now
  an alias for "latest" which is weird

This change does lead to some thrashing when applying styles,
so I'm not going to do that across the board right now. Changes include:
- extra spaces in lambda-captures
- nicer alignment of lambda-bodies