games/jchessboard: fix build with jdk21
JDK21 is more strict in the encoding of source files. It does not ignore comments anymore.
[javac] VirtualBoard.java:1174: error: unmappable character (0xF6) for encoding UTF-8
[javac] // utest.pgn: Qe4 oder Q3e4? (Q5e4 ist eigentlich nicht m?glich.)
PR: 291663
Approved-By: maintainer timeout