[JENKINS-69044] Adapt to executable WAR changes in JenkinsRule

[basil]

See JENKINS-69044. WarExploder contains two code paths that do the same thing: a standard code path for Maven, and a fallback code path for IntelliJ. The standard code path for Maven hunts down the WAR file by looking for winstone.jar and a class in the executable package. Prior to jenkinsci/jenkins#6706 this was working as intended with winstone.jar in the root of the WAR file and the class in the executable package being the placeholder executable.Executable, but jenkinsci/jenkins#6706 moved winstone.jar into the executable package and got rid of the placeholder class executable.Executable in favor of the real Main class (previously in the unnamed package but moved to the executable package in jenkinsci/jenkins#6706). This broke the primary Maven code path, but this went unnoticed because we went into the secondary IntelliJ code path. However the secondary IntelliJ code path does not seem to work for Gradle plugins as reported in JENKINS-69044.

This PR corrects the problem by getting adapting the primary Maven code path to work with the changes from jenkinsci/jenkins#6706: looking for winstone.jar in the executable package and looking for the real executable.Main instead of the placeholder executable.Executable if we are running with jenkinsci/jenkins#6706. The old code is left for compatibility but separated into an if statement that will be easy to delete later on when we eventually drop support.

To test this, I stepped through four scenarios in the debugger:

• Maven, 2.357: Verified that we went into the primary Maven code path before this change and continued to do so after this change.
• Maven, 2.358: Verified that before this change we erroneously went into the secondary IntelliJ code path, but after this change went into the primary Maven code path as expected.
• IntelliJ, 2.357: Verified that we went into the secondary IntelliJ code path before and after this change.
• IntelliJ, 2.358: Verified that we went into the secondary IntelliJ code path before and after this change.