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[bot]]

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 :warning:

Additional details and impacted files

```diff @@ 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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