Spring Framework now uses a multi-release jar for its virtual thread
support. If the shaded jar is not marked as being a multi-release jar
and virtual threads are enabled, the application will fail to start as
the Java 21+ virtual threads delegate will be unavailable.
This commit updates the starter parent's default configuration for the
Maven Shade plugin to set Multi-Release: true in the application's
manifest.
Closes gh-43284
d924e4d999