sysutils/jvmtop: pin to java 8

It misses tools.jar when building with jdk11+.

[ERROR] Failed to execute goal on project jvmtop: Could not resolve dependencies for project com.jvmtop:jvmtop:jar:0.9.0-SNAPSHOT: Could not find artifact com.sun:tools:jar:1.6 at specified path /usr/local/openjdk11/../lib/tools.jar -> [Help 1]

NB: upstream repo is inactive for 9 years. A fork (https://github.com/MOschIT/jvmtop)
claims jdk21 support, but that is outside of the scope of the issue I'm trying to solve.

PR: 291538
Approved-By: maintainer timeout