Integrated: 8336106: Genshen: Fix use of missing API in Shenandoah Old Heuristic test

[William Kemper]

The recent fixes for the Shenandoah Old Heuristic test used an API that hasn't been backported. This change uses a different API available in 21 to achieve the same end.

-------------

Commit messages:
 - Fix old heuristic test for 21

Changes: https://git.openjdk.org/shenandoah-jdk21u/pull/65/files
  Webrev: https://webrevs.openjdk.org/?repo=shenandoah-jdk21u&pr=65&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8336106
  Stats: 1 line in 1 file changed: 0 ins; 0 del; 1 mod
  Patch: https://git.openjdk.org/shenandoah-jdk21u/pull/65.diff
  Fetch: git fetch https://git.openjdk.org/shenandoah-jdk21u.git pull/65/head:pull/65

PR: https://git.openjdk.org/shenandoah-jdk21u/pull/65