Fix multi-release JAR issue with VirtualThreadDelegate

This commit ensures that both `VirtualThreadDelegate` implementations
expose the same public API. If not, JAR verification fails with the
following message:

```
jar --validate --file spring-core-6.2.13-SNAPSHOT.jar
entry: META-INF/versions/21/org/springframework/core/task/VirtualThreadDelegate.class, contains a class with different api from earlier version
```

Fixes gh-35773

Author: bclozel

spring-core/src/main/java21/org/springframework/core/task/VirtualThreadDelegate.java

@@ -30,6 +30,10 @@ final class VirtualThreadDelegate {
 
 	private final Thread.Builder threadBuilder = Thread.ofVirtual();
 
+	public VirtualThreadDelegate() {
+
+	}
+
 	public ThreadFactory virtualThreadFactory() {
 		return this.threadBuilder.factory();
 	}