CMS-11148 Update to hippo htmldiff library 2.0.0 which no longer uses nekohtml...
CMS-11148 Update to hippo htmldiff library 2.0.0 which no longer uses nekohtml but htmlcleaner (backported from CMS-11122) Dropping nekohtml also required replacing its direct usage in HtmlValidator and HtmlDiffModelTest with htmlcleaner. Also replaced deprecated (Xerces provided) SAXSerializer usage for Console XML Content Export with standard JAXP XMLTransformer. Finally removed now no longer needed xerces dependencies (cherry picked from commit 411d2bf1)
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