fix: correct Codeforces language version IDs and default to C++20 (#349)

## 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`.

lua/cp/race.lua

@@ -61,9 +61,10 @@ local function refetch_start_time()
   local new_time = cache.get_contest_start_time(race_state.platform, race_state.contest_id)
   if new_time and new_time ~= race_state.start_time then
     race_state.start_time = new_time
-    race_state.contest_name =
-      cache.get_contest_display_name(race_state.platform, race_state.contest_id)
-      or race_state.contest_id
+    race_state.contest_name = cache.get_contest_display_name(
+      race_state.platform,
+      race_state.contest_id
+    ) or race_state.contest_id
   end
 end
 
@@ -121,11 +122,7 @@ local function race_try_setup(platform, contest_id, language, attempt, token)
       end
       if attempt >= MAX_RETRY_ATTEMPTS then
         logger.log(
-          ('Failed to load %s contest "%s" after %d attempts'):format(
-            display,
-            contest_id,
-            attempt
-          ),
+          ('Failed to load %s contest "%s" after %d attempts'):format(display, contest_id, attempt),
           { level = vim.log.levels.ERROR }
         )
         return