Update Catch2 library settings for MSVC and C++ standard

[AbhishekkSaini]

Description

Improves Catch2 compilation times by enabling some standard build optimizations that were previously disabled.

The main changes are:

• Turn on MSVC's /MP flag to compile files in parallel across CPU cores
• Use C++17 instead of defaulting to C++20 (MSVC wastes time scanning for modules in C++20 mode even though we don't use them)
• Enable unity builds with a batch size of 8 files per unit

I tested a few different batch sizes and found that 8 works well - higher values like 25 caused some operator overload ambiguity issues during compilation, but 8 seems to avoid those while still giving good speedup.

On my machine (8 cores, MSVC), build time went from about 70 seconds down to around 10-15 seconds, so it's a pretty substantial improvement. The /MP flag only affects MSVC builds, so other compilers shouldn't see any difference there, but they should still benefit from the unity build settings.

GitHub Issues

Fixes #2894

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 91.09%. Comparing base (de7e863) to head (94ef6c4).

Additional details and impacted files

@@            Coverage Diff             @@
##            devel    #3066      +/-   ##
==========================================
- Coverage   91.14%   91.09%   -0.06%     
==========================================
  Files         202      202              
  Lines        8831     8840       +9     
==========================================
+ Hits         8049     8052       +3     
- Misses        782      788       +6     

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