[TASK] Use csConv API methods

There are several places in the core that use
TYPO3's homebrew charset conversion over the configured mbstring/iconv selection.

The change modifies the places to use the API methods
instead.

Resolves: #72590
Releases: master
Change-Id: Ibf8ed5e810d37c301db2b7e3f6e94b76a1061101
Reviewed-on: https://review.typo3.org/45685


Reviewed-by: Andreas Fernandez <typo3@scripting-base.de>
Reviewed-by: Wouter Wolters <typo3@wouterwolters.nl>
Tested-by: Wouter Wolters <typo3@wouterwolters.nl>
Reviewed-by: Morton Jonuschat <m.jonuschat@mojocode.de>
Tested-by: Morton Jonuschat <m.jonuschat@mojocode.de>