Revert "Update ant to v1.10.16"

Fix a Windows bug introduced by the update to Apache Ant 1.10.16. The unreleased Apache Ant 1.10.17 What's New describes bug 69992 as:

The JavaEnvUtils and FileUtils classes statically depend on each other since Ant 1.10.16 making it impossible to load JavaEnvUtils without loading FileUtils first causing NullPointerException for programmatic use on Windows. This affected Eclipse and may also affect other projects using Ant as a library.

The Eclipse project discusses it in detail in eclipse-platform/eclipse.platform#2605 (comment)

Automated tests on ci.jenkins.io Windows agents have been failing since 3 Apr 2026. The failures report a stack trace:

Caused by: java.lang.NullPointerException: Cannot invoke "org.apache.tools.ant.util.DeweyDecimal.compareTo(org.apache.tools.ant.util.DeweyDecimal)" because "org.apache.tools.ant.util.JavaEnvUtils.parsedJavaVersion" is null
	at org.apache.tools.ant.util.JavaEnvUtils.isAtLeastJavaVersion(JavaEnvUtils.java:301)
	at org.apache.tools.ant.util.FileUtils.<clinit>(FileUtils.java:94)

The failure only happens on Windows because the Ant source code added additional logic for Windows junctions in commit:

• apache/ant@45b84bb#diff-fdf66b5ecb8a9eb6a9d4e8bb1bc95ce8fc8872c5664fa7e5d899b3dc31af2891R91

This reverts commit d8ce08e from pull request:

• Update dependency org.apache.ant:ant to v1.10.16 #26566

This is another case where the subset of tests run on Windows missed a test failure. It is even more surprising that the Windows test selection process did not identify that the failing tests should be run in all pull requests after the initial failure on the master branch. Probably needs further discussion with the predictive test selection experts at CloudBees. @daniel-beck you are not alone.

This is also a case where the acceptance test harness and the plugin BOM did not catch the issue, because they do not run on Windows. This type of platform specific bug happens so infrequently that I don't think it is worth the expense to run those tests on Windows.

Testing done

• Confirmed that I can duplicate the problem on my Windows computers with the commands:
  mvn -am -pl war,bom -Pquick-build clean install
mvn -pl test -Dtest=hudson.slaves.JNLPLauncherRealTest test
• Confirmed that reverting Apache Ant 1.10.16 fixes the issue

I was unable to duplicate the issue from a Windows controller using websocket connected agents from a Windows computer or from two different Linux computers. Likewise I was unable to duplicate the issue from a Windows controller with a Linux agent that connected without using websocket. I suspect there is a user visible bug in Jenkins 2.558 due to this issue, but I was unable to find it.

Screenshots (UI changes only)

Before

After

Proposed changelog entries

• Fix Windows null pointer exception due to Apache Ant 1.10.16 (regression in 2.558).

Proposed changelog category

/label bug

Proposed upgrade guidelines

N/A

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Desired reviewers

@jenkinsci/core-pr-reviewers I'd appreciate a review of this pull request so that the regression is fixed in the next weekly release.

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