Fix JsonContentAssert type safety warnings

Previously, JsonContentAssert returns AbstractMapAssert from
extractingJsonPathMapValue. This could lead to type safety warnings
when calling one of the assert's methods with a generic varargs
parameter such as
contains(Entry<? extends Object, ? extends Object>...).

This commit replaces the use of both AbstractMapAssert and
AbstractListAssert with MapAssert and ListAssert respectively. These
classes use final methods and @SafeVargs args to prevent the
above-described problem from occurring.

See gh-9675

spring-boot-test/src/main/java/org/springframework/boot/test/json/JsonContentAssert.java

@@ -26,12 +26,11 @@ import com.jayway.jsonpath.JsonPath;
 import org.assertj.core.api.AbstractAssert;
 import org.assertj.core.api.AbstractBooleanAssert;
 import org.assertj.core.api.AbstractCharSequenceAssert;
-import org.assertj.core.api.AbstractListAssert;
-import org.assertj.core.api.AbstractMapAssert;
 import org.assertj.core.api.AbstractObjectAssert;
 import org.assertj.core.api.Assert;
 import org.assertj.core.api.Assertions;
-import org.assertj.core.api.ObjectAssert;
+import org.assertj.core.api.ListAssert;
+import org.assertj.core.api.MapAssert;
 import org.skyscreamer.jsonassert.JSONCompare;
 import org.skyscreamer.jsonassert.JSONCompareMode;
 import org.skyscreamer.jsonassert.JSONCompareResult;
@@ -947,11 +946,12 @@ public class JsonContentAssert extends AbstractAssert<JsonContentAssert, CharSeq
 	 * @param expression the {@link JsonPath} expression
 	 * @param args arguments to parameterize the {@code JsonPath} expression with, using
 	 * formatting specifiers defined in {@link String#format(String, Object...)}
+	 * @param <E> element type
 	 * @return a new assertion object whose object under test is the extracted item
 	 * @throws AssertionError if the path is not valid or does not result in an array
 	 */
 	@SuppressWarnings("unchecked")
-	public AbstractListAssert<?, ?, Object, ObjectAssert<Object>> extractingJsonPathArrayValue(
+	public <E> ListAssert<E> extractingJsonPathArrayValue(
 			CharSequence expression, Object... args) {
 		return Assertions.assertThat(
 				extractingJsonPathValue(expression, args, List.class, "an array"));
@@ -961,12 +961,14 @@ public class JsonContentAssert extends AbstractAssert<JsonContentAssert, CharSeq
 	 * Extract the map value at the given JSON path for further object assertions.
 	 * @param expression the {@link JsonPath} expression
 	 * @param args arguments to parameterize the {@code JsonPath} expression with, using
+	 * @param <K> key type
+	 * @param <V> value type
 	 * formatting specifiers defined in {@link String#format(String, Object...)}
 	 * @return a new assertion object whose object under test is the extracted item
 	 * @throws AssertionError if the path is not valid or does not result in a map
 	 */
 	@SuppressWarnings("unchecked")
-	public AbstractMapAssert<?, ?, Object, Object> extractingJsonPathMapValue(
+	public <K, V> MapAssert<K, V> extractingJsonPathMapValue(
 			CharSequence expression, Object... args) {
 		return Assertions.assertThat(
 				extractingJsonPathValue(expression, args, Map.class, "a map"));