Revert nullability changes in reactive transaction support

After further consideration, we have decided to revert the
nullability changes in the reactive TransactionCallback and
TransactionalOperator.

See gh-35561

spring-tx/src/main/java/org/springframework/transaction/reactive/TransactionCallback.java

@@ -16,7 +16,6 @@
 
 package org.springframework.transaction.reactive;
 
-import org.jspecify.annotations.Nullable;
 import org.reactivestreams.Publisher;
 
 import org.springframework.transaction.ReactiveTransaction;
@@ -37,7 +36,7 @@ import org.springframework.transaction.ReactiveTransaction;
  * @see TransactionalOperator
  */
 @FunctionalInterface
-public interface TransactionCallback<T extends @Nullable Object> {
+public interface TransactionCallback<T> {
 
 	/**
 	 * Gets called by {@link TransactionalOperator} within a transactional context.

spring-tx/src/main/java/org/springframework/transaction/reactive/TransactionalOperator.java

@@ -16,7 +16,6 @@
 
 package org.springframework.transaction.reactive;
 
-import org.jspecify.annotations.Nullable;
 import reactor.core.publisher.Flux;
 import reactor.core.publisher.Mono;
 
@@ -60,7 +59,7 @@ public interface TransactionalOperator {
 	 * @throws TransactionException in case of initialization, rollback, or system errors
 	 * @throws RuntimeException if thrown by the TransactionCallback
 	 */
-	default <T extends @Nullable Object> Flux<T> transactional(Flux<T> flux) {
+	default <T> Flux<T> transactional(Flux<T> flux) {
 		return execute(it -> flux);
 	}
 
@@ -71,7 +70,7 @@ public interface TransactionalOperator {
 	 * @throws TransactionException in case of initialization, rollback, or system errors
 	 * @throws RuntimeException if thrown by the TransactionCallback
 	 */
-	default <T extends @Nullable Object> Mono<T> transactional(Mono<T> mono) {
+	default <T> Mono<T> transactional(Mono<T> mono) {
 		return execute(it -> mono).singleOrEmpty();
 	}
 
@@ -86,7 +85,7 @@ public interface TransactionalOperator {
 	 * @throws TransactionException in case of initialization, rollback, or system errors
 	 * @throws RuntimeException if thrown by the TransactionCallback
 	 */
-	<T extends @Nullable Object> Flux<T> execute(TransactionCallback<T> action) throws TransactionException;
+	<T> Flux<T> execute(TransactionCallback<T> action) throws TransactionException;
 
 
 	// Static builder methods