[TASK] Change visibility of CharsetConverter init methods

Various CharsetConverter methods are there for initialization conversion,
which are marked as @private and are only used within the class.

The patch switches the method signatures from public to protected.

The following methods are affected:
- initCharset()
- initUnicodeData()
- initCaseFolding()
- initToASCII()

Resolves: #85116
Releases: master
Change-Id: I153123068cbfbf0baee729aab7dc0946913d5d7c
Reviewed-on: https://review.typo3.org/57088


Reviewed-by: Mathias Brodala <mbrodala@pagemachine.de>
Tested-by: TYPO3com <no-reply@typo3.com>
Reviewed-by: Georg Ringer <georg.ringer@gmail.com>
Tested-by: Georg Ringer <georg.ringer@gmail.com>
Reviewed-by: Wouter Wolters <typo3@wouterwolters.nl>
Tested-by: Mathias Brodala <mbrodala@pagemachine.de>
Reviewed-by: Jan Helke <typo3@helke.de>
Tested-by: Jan Helke <typo3@helke.de>