springboot Fix multi-release JAR test on JDK 15

Hi,

I just did a quick for #21604 to see if everything works on JDK 15 with the custom buildJavaHome property I introduced for JDK 14.

This PR fixes a test failure in JarFileTests.multiReleaseEntry.

I'm wondering if we should add some entries in advance already for JDK 16 & 17 ;-)

Cheers, Christoph

Comment From: snicoll

I'm wondering if we should some entries in advance already for JDK 16 & 17 ;-)

Ah! I don't think we should do that as we'll have to do a check every time we need to support a new JDK anyway.

Comment From: dreis2211

I agree - I just wanted to mention it at least once. ;)