Use ICU's C API only

[rmisev]

What is done:

• Replace the C++ ICU function icu::toUCharPtr with the locally implemented to_UChar_ptr function.
• Use u_getVersion function to get ICU major version

Benefits:

• Using ICU's C API only allows the program to run with a different version of ICU than the compiled one.
• ICU 75.1 C++ API requires C++17. When using the C API only, a C++11 or C++14 compiler can be used.

See also: https://unicode-org.github.io/icu/userguide/icu4c/build.html#icu-as-a-system-level-library

[codecov]

Codecov Report

Attention: Patch coverage is 83.33333% with 2 lines in your changes missing coverage. Please review.

Project coverage is 99.65%. Comparing base (0877b18) to head (b873487).

Files	Patch %	Lines
src/url_idna.cpp	83.33%	2 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main      #47      +/-   ##
==========================================
- Coverage   99.74%   99.65%   -0.09%     
==========================================
  Files          16       16              
  Lines        2322     2333      +11     
  Branches      386      387       +1     
==========================================
+ Hits         2316     2325       +9     
- Misses          6        8       +2     

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