[MISC] Remove std::ranges::copy and std::cpp20::back_inserter.

[smehringer]

Follow up of #44

Most changes are euqivalent because we don't need e.g. std::ranges::find if we have a common range anyway. IMHO this change even became more readable.

The only not nice change is this one

+    std::string str = [&range] () { std::string s; for (auto c : range) s.push_back(c); return s; }();
-    std::string str;
-    std::ranges::copy(range, std::cpp20::back_inserter(str));

because it is in the snippets and thus code that is exposed to the user.
We might want to think about adding the std/ranges module to the sharg parser or drop gcc-9 support which would make #include <ranges> available directly. Then we can change this back.

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[codecov]

Codecov Report

Merging #54 (877b5ea) into master (a3dedd1) will increase coverage by 0.01%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master      #54      +/-   ##
==========================================
+ Coverage   95.22%   95.23%   +0.01%     
==========================================
  Files          13       13              
  Lines         921      923       +2     
==========================================
+ Hits          877      879       +2     
  Misses         44       44              

Impacted Files	Coverage Δ
include/sharg/argument_parser.hpp	97.53% <100.00%> (ø)
include/sharg/detail/format_help.hpp	89.47% <100.00%> (+0.18%)	⬆️
include/sharg/validators.hpp	100.00% <100.00%> (ø)

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[Irallia]

LGFM, thank you!