The backport for https://bugs.openjdk.org/browse/JDK-8351038 requires JDK 19 or later (ExecutorService needs to extend AutoCloseable).
The build.properties for jfx21u have JDK17 as minimum build version, which does not meet this criteria.
For recent JavaFX versions, we have the minimum JDK to be JavaFX version minus 1, but since we didn't have this rule when JavaFX 21 was released, we should not assume this in jfx21u.