java/sigar: fix running with openjdk8

Running Cassandra 3 gave:
java.lang.UnsupportedClassVersionError: org/hyperic/sigar/SigarException has been compiled by a more recent version of the Java Runtime (class file version 65.0), this version of the Java Runtime only recognizes class file versions up to 52.0

PR: 294323
Reported-by: Lapo Luchini
Fixes: f3a12dc72aa72af22212e3e37c5f29652eae0cbb "fix compilation with jdk21"
MFH: 2026Q2
Pointy-hat: to me