Revise nullability for Core Retry after upgrade to NullAway 0.12.10

This commit revises the nullability declarations in Retryable,
RetryOperations, and RetryTemplate after the upgrade to NullAway 0.12.10.

See gh-35492

spring-context/src/main/java/org/springframework/resilience/retry/AbstractRetryInterceptor.java

@@ -101,7 +101,7 @@ public abstract class AbstractRetryInterceptor implements MethodInterceptor {
 		RetryTemplate retryTemplate = new RetryTemplate(retryPolicy);
 
 		try {
-			return retryTemplate.execute(new Retryable<>() {
+			return retryTemplate.execute(new Retryable<@Nullable Object>() {
 				@Override
 				public @Nullable Object execute() throws Throwable {
 					return (invocation instanceof ProxyMethodInvocation pmi ?

spring-core/src/main/java/org/springframework/core/retry/RetryOperations.java

@@ -46,6 +46,6 @@ public interface RetryOperations {
 	 * @return the result of the {@code Retryable}, if any
 	 * @throws RetryException if the {@code RetryPolicy} is exhausted
 	 */
-	<R> @Nullable R execute(Retryable<? extends @Nullable R> retryable) throws RetryException;
+	<R extends @Nullable Object> R execute(Retryable<R> retryable) throws RetryException;
 
 }

spring-core/src/main/java/org/springframework/core/retry/RetryTemplate.java

@@ -134,7 +134,7 @@ public class RetryTemplate implements RetryOperations {
 	 * @throws RetryException if the {@code RetryPolicy} is exhausted
 	 */
 	@Override
-	public <R> @Nullable R execute(Retryable<? extends @Nullable R> retryable) throws RetryException {
+	public <R extends @Nullable Object> R execute(Retryable<R> retryable) throws RetryException {
 		String retryableName = retryable.getName();
 		// Initial attempt
 		try {

spring-core/src/main/java/org/springframework/core/retry/Retryable.java

@@ -31,14 +31,14 @@ import org.jspecify.annotations.Nullable;
  * @see RetryOperations
  */
 @FunctionalInterface
-public interface Retryable<R> {
+public interface Retryable<R extends @Nullable Object> {
 
 	/**
 	 * Method to execute and retry if needed.
 	 * @return the result of the operation
 	 * @throws Throwable if an error occurs during the execution of the operation
 	 */
-	@Nullable R execute() throws Throwable;
+	R execute() throws Throwable;
 
 	/**
 	 * A unique, logical name for this retryable operation, used to distinguish