Speed up C++ CI and cover GCC 8–11 compatibility

[vincentlaucsb]

C++ CI spends most of its time compiling serially: on the recent PR #328 run, GCC C++20 spent 314 seconds building versus 9 seconds testing, while Windows C++20 spent 430 seconds building versus 15 seconds testing. Bound CMake builds to two concurrent jobs, cancel superseded PR runs, shallow-fetch Catch2, and remove unnecessary compiler/Catch2 setup. All existing compiler matrices, build targets, and test commands remain intact. Also correct the existing CodeQL languages input typo found by actionlint.

Add focused C++17 compatibility checks with GCC 8.5, 9.5, 10.5, and 11.1. These build and run floating-point field conversions against both csv and csv_no_simd, the canonical generated single header, and its explicit floating-from_chars opt-out. Old compilers run in pinned-version Docker images with checksum-verified portable CMake; no Catch2 or data submodule is required. Generated-header diagnostics still run after a linked-library failure.

This PR is separate from #328 and intentionally does not include its parser fix. GCC 8–10 are expected to fail on the existing unsupported floating-point std::from_chars path until that fix lands. The new jobs are strict: no continue-on-error or expected-failure masking.

Validation: actionlint 1.7.12 and git diff checks passed. Generated the canonical single header, then compiled and ran all four smoke checks locally with MSVC in C++17 mode. Historical GCC container execution awaits CI; Docker is not running locally. Actual speedup remains to be measured after CI runs.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 93.64%. Comparing base (39b6335) to head (4d3d57d).

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #329      +/-   ##
==========================================
+ Coverage   91.58%   93.64%   +2.05%     
==========================================
  Files          55        6      -49     
  Lines        3541      236    -3305     
  Branches     1043        0    -1043     
==========================================
- Hits         3243      221    -3022     
+ Misses        115       15     -100     
+ Partials      183        0     -183     

Flag	Coverage Δ
cpp	?
python	93.64% <ø> (ø)

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