[TASK] Deprecate various functionality in CharsetConverter

Due to native support and a polyfill for mb_string(), lot of functionality of
CharsetConverter was simplified throughout TYPO3 in the last release
to ensure faster charset conversion.

Many methods and properties within CharsetConverter can be deprecated
now:
- CharsetConverter->synonyms
- CharsetConverter->parse_charset()
- Fourth parameter of CharsetConverter->conv()
- CharsetConverter->convArray()
- CharsetConverter->utf8_to_entities()
- CharsetConverter->entities_to_utf8()
- CharsetConverter->crop()
- CharsetConverter->convCaseFirst()
- CharsetConverter->utf8_char2byte_pos()

Additionally the following public properties have been changed to have a "protected"
visibility, as this only reflects internal state:
- CharsetConverter->noCharByteVal
- CharsetConverter->parsedCharsets
- CharsetConverter->toASCII
- CharsetConverter->twoByteSets
- CharsetConverter->eucBasedSets

Resolves: #85122
Releases: master
Change-Id: Ic67dd1cffa36cd625c844ae82fdca8447a317efe
Reviewed-on: https://review.typo3.org/57094


Tested-by: TYPO3com <no-reply@typo3.com>
Reviewed-by: Wouter Wolters <typo3@wouterwolters.nl>
Tested-by: Wouter Wolters <typo3@wouterwolters.nl>
Reviewed-by: Christian Kuhn <lolli@schwarzbu.ch>
Tested-by: Christian Kuhn <lolli@schwarzbu.ch>