## Problem
`LANGUAGE_VERSIONS` mapped CF `c++17` to `89` (actually GNU G++20) and
defaulted to `c++17`, which didn't match any CF entry. Helpdocs listed
CF as having only `c++17`.
## Solution
Fix CF version map to `c++17 → 54`, `c++20 → 89`, `c++23 → 91`, add
`pypy3 → 70`. Change `DEFAULT_VERSIONS.cpp` to `c++20` (no behavioral
change — `89` was already the submit default). Update helpdocs. Fix
StyLua formatting in `race.lua`.