[8.x] [Test] Increase test secret key length for AwsStsHttpHandler

The initial backport elastic#117738 did not contain the change for
AwsStsHttpHandler from elastic#117675. This PR adds it.

Backport of elastic#117675

test/fixtures/aws-sts-fixture/src/main/java/fixture/aws/sts/AwsStsHttpHandler.java

@@ -28,6 +28,7 @@
 import java.util.stream.Collectors;
 
 import static org.elasticsearch.test.ESTestCase.randomIdentifier;
+import static org.elasticsearch.test.ESTestCase.randomSecretKey;
 
 /**
  * Minimal HTTP handler that emulates the AWS STS server
@@ -102,7 +103,7 @@ public void handle(final HttpExchange exchange) throws IOException {
                     ROLE_ARN,
                     ROLE_NAME,
                     sessionToken,
-                    randomIdentifier(),
+                    randomSecretKey(),
                     ZonedDateTime.now().plusDays(1L).format(DateTimeFormatter.ofPattern("yyyy-MM-dd'T'HH:mm:ssZ")),
                     accessKey
                 ).getBytes(StandardCharsets.UTF_8);