docs: require Java 8 as the active JDK

[tomatotomata]

Description

Fixes #9111.

Clarify that JBMC requires Java 8 to be the active default JDK, not merely installed alongside a newer JDK. The Linux instructions now show runtime/compiler verification and the appropriate alternatives commands for Debian-like and Red Hat/Fedora systems. The FreeBSD instructions now explain how to activate OpenJDK 8 through JAVA_HOME and PATH.

Validation

• git diff --check

This documentation-only change does not require a compiled CBMC/JBMC test run.

[kroening]

Fixes #9111.

Clarify that JBMC requires Java 8 to be the active default JDK, not merely installed alongside a newer JDK. The Linux instructions now show runtime/compiler verification and the appropriate alternatives commands for Debian-like and Red Hat/Fedora systems. The FreeBSD instructions now explain how to activate OpenJDK 8 through JAVA_HOME and PATH.

Can we instead not require Java 8 to be the default JDK?

[tomatotomata]

Yes. The build uses the Java toolchain available in its shell, so requiring Java 8 as the system-wide default was unnecessarily invasive. c526747 now instructs users to select Java 8 only for the build via JAVA_HOME and PATH, then verify java and javac report 1.8.