We should update jtreg to use Ant 1.10.15

Older versions of Ant fail to run with new JDKs, and print for example:

Error occurred during initialization of VM
java.lang.Error: A command line option has attempted to allow or enable the Security Manager. Enabling a Security Manager is not supported.
at java.lang.System.initPhase3(java.base@24.0.1/System.java:1947)