clean: port away from QTextCodec in slob.cc and clean up iconv

[shenlebantongying]

Another attempt to remove usage of Core5Compat.

Is this change correct?

The Qt5 version of QTextCodec::codecForName underneath uses ICU

GNU libiconv has a large table of encoding alias

https://git.savannah.gnu.org/gitweb/?p=libiconv.git;a=blob;f=lib/encodings.def;h=e5063d329886818033e6d3aa14618c75d8b07852;hb=HEAD#l441

ICU also has a big table of encoding alias

https://github.com/unicode-org/icu/blob/1414e80f6c1376afa65eb388aaaafb53169fc3dc/icu4c/source/data/mappings/convrtrs.txt#L891

Both includes names from various sources like INNA, glibc, java, windows, so I assume QTextCodec::codecForName and iconv_open are equal.

unrelated

QStringConverter since Qt6.5 also uses ICU to support equal set of encodings like old QTextCodec btw.

Maybe the correct path forward is just use ICU instead, because multiple dependencies of GD already depends on it one way or another.

[shenlebantongying]

Also, the original code appears to be mem leaking.

[xiaoyifang]

USE_ICONV can be removed and default to true

[sonarcloud]

Quality Gate failed

Failed conditions
0.0% Coverage on New Code (required ≥ 80%)
C Reliability Rating on New Code (required ≥ A)
B Maintainability Rating on New Code (required ≥ A)

See analysis details on SonarCloud

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