Add preview annotation to Struct/UnionLayout

Author: mcimadamore

src/java.base/share/classes/java/lang/foreign/PaddingLayout.java

@@ -26,14 +26,18 @@
 package java.lang.foreign;
 
 import jdk.internal.foreign.layout.PaddingLayoutImpl;
+import jdk.internal.javac.PreviewFeature;
 
 /**
  * A padding layout. A padding layout specifies the size of extra space which is typically not accessed by applications,
  * and is typically used for aligning member layouts around word boundaries.
  *
  * @implSpec
  * Implementing classes are immutable, thread-safe and <a href="{@docRoot}/java.base/java/lang/doc-files/ValueBased.html">value-based</a>.
+ *
+ * @since 20
  */
+@PreviewFeature(feature=PreviewFeature.Feature.FOREIGN)
 public sealed interface PaddingLayout extends MemoryLayout permits PaddingLayoutImpl {
 
     /**

src/java.base/share/classes/java/lang/foreign/StructLayout.java

@@ -34,7 +34,7 @@
  * @implSpec
  * Implementing classes are immutable, thread-safe and <a href="{@docRoot}/java.base/java/lang/doc-files/ValueBased.html">value-based</a>.
  *
- * @since 19
+ * @since 20
  */
 @PreviewFeature(feature=PreviewFeature.Feature.FOREIGN)
 public sealed interface StructLayout extends GroupLayout permits StructLayoutImpl {

src/java.base/share/classes/java/lang/foreign/UnionLayout.java

@@ -34,7 +34,7 @@
  * @implSpec
  * Implementing classes are immutable, thread-safe and <a href="{@docRoot}/java.base/java/lang/doc-files/ValueBased.html">value-based</a>.
  *
- * @since 19
+ * @since 20
  */
 @PreviewFeature(feature=PreviewFeature.Feature.FOREIGN)
 public sealed interface UnionLayout extends GroupLayout permits UnionLayoutImpl {