Mark shaded application's jar as multi-release by default

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

Author: wilkinsona

spring-boot-project/spring-boot-starters/spring-boot-starter-parent/build.gradle

@@ -234,7 +234,10 @@ publishing.publications.withType(MavenPublication) {
 											}
 											transformer(implementation: 'org.apache.maven.plugins.shade.resource.ServicesResourceTransformer')
 											transformer(implementation: 'org.apache.maven.plugins.shade.resource.ManifestResourceTransformer') {
-												delegate.mainClass('${start-class}')
+												manifestEntries {
+													delegate.'Main-Class'('${start-class}')
+													delegate.'Multi-Release'('true')
+												}
 											}
 										}
 									}