Fix predicate-not modern example to compile on Java 11

[Copilot]

The streams/predicate-not modern snippet used Stream.toList(), which is only available in Java 16+, while the pattern is marked as JDK 11. This PR aligns the example with Java 11-compatible stream collection syntax.

• Scope

  • Updated the modern code example in content/streams/predicate-not.yaml.
  • Updated the corresponding proof file proof/streams/PredicateNot.java to keep it in sync.

• Code change

  • Replaced Java 16+ terminal operation with Java 11-compatible collector:

  List<String> nonEmpty = list.stream()
  .filter(Predicate.not(String::isBlank))
  .collect(Collectors.toList());

• Compatibility impact

  • Preserves the intended focus of the pattern (Predicate.not(...) introduced in Java 11) while removing the accidental dependency on newer Stream APIs.

• Fixes[Code Issue] Predicate.not()による否定 #158