The UnicodeReport.jrxml file on the master branch works with JasperReports 7.x.
If you want the report that works with JasperReports 6.21.0 you can see the file as present on the 6.21.0 tag: https://github.com/TIBCOSoftware/jasperreports/blob/6.21.0/jasperreports/demo/samples/unicode/reports/UnicodeReport.jrxml