Improve `specialCharFilter` implementation

[bastianjoel]

The implementation of specialCharFilter might need some improvement.

openslides-search-service/pkg/search/textindex.go

Lines 124 to 127 in 680ff8a

	func (f *specialCharFilter) Filter(input []byte) []byte {
	input = []byte(html.UnescapeString(string(input)))
	return input
	}