Fix spring-core multirelease jar

[Sineaggi]

The multi-release jar for spring-core doesn't validate when running jar --validate --file spring-core.jar. Instead, the following warnings are printed.

jar --validate --file /Users/cwalker/public-repos/spring-framework/spring-core/build/libs/spring-core-7.0.0-SNAPSHOT.jar
entry: META-INF/versions/21/org/springframework/core/task/VirtualThreadDelegate.class, contains a class with different api from earlier version
entry: META-INF/versions/24/org/springframework/core/type/classreading/ClassFileMetadataReaderFactory.class, contains a new public class not found in base entries

These can be fixed by adding an empty constructor to VirtualThreadDelegate and by removing the public modifier in ClassFileMetadataReaderFactory.

[Sineaggi]

I've added an example fix + multi-release jar validation task in this pr #35768.

[bclozel]

Superseded by the PR