fix: align documented build prerequisites

[dkropachev]

Summary

• document Java 8 as the runtime target and JDK 11 as the source-build requirement
• raise the documented Maven minimum from 3.3 to 3.6.3 to match the current Compiler and Surefire plugins
• enforce both build-tool minimums during validate, before the Java-11-only license plugin runs
• keep generated artifacts targeting Java 8

Closes #12.

Compatibility

This changes only the toolchain required to build the Simulacron repository. Published artifacts remain Java 8 bytecode, so Maven consumers such as scylladb/java-driver are unaffected.

Validation

• mvn -B test -Dfmt.skip=true with JDK 11: passed
• mvn -q package -DskipTests -Dfmt.skip=true with JDK 11: passed
• packaged standalone JAR --help under Java 8: passed
• mvn validate with JDK 8: rejected early with the intended Java requirement message
• Maven 3.3.9 + JDK 11 mvn validate: rejected early with the intended Maven requirement message

A full integration-test run was also attempted. The unrelated HttpPrimeQueryIntegrationTest prepared-query race reproduced twice in different test cases; the regular CI unit-test lifecycle passes.