fix: __cplusplus check for <version> header

[tearfur]

<version> is a standard C++20 feature, so I figured the correct __cplusplus value should be 202002L.

In practice, this PR fixes a compiler warning when compiling on C++20 on g++15:

In file included from /transmission/third-party/wide-integer/math/wide_integer/uintwide_t.h:17,
                 from /transmission/libtransmission/peer-mse.cc:12:
/usr/include/c++/15/ciso646:49:6: warning: #warning "<ciso646> is not a standard header since C++20, use <version> to detect implementation-specific macros" [-Wcpp]
   49 | #    warning "<ciso646> is not a standard header since C++20, use <version> to detect implementation-specific macros"
      |      ^~~~~~~

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 100.0%. Comparing base (aed8a39) to head (59f48b0).
⚠️ Report is 2 commits behind head on master.

Additional details and impacted files

@@           Coverage Diff           @@
##           master     #473   +/-   ##
=======================================
  Coverage   100.0%   100.0%           
=======================================
  Files          41       41           
  Lines        3772     3769    -3     
  Branches      320      320           
=======================================
- Hits         3772     3769    -3     

Files with missing lines	Coverage Δ
math/wide_integer/uintwide_t.h	100.0% <ø> (ø)

... and 2 files with indirect coverage changes

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[ckormanyos]

Oh I think I forgot this. Let me get it done.