Update google-auth-library-java/oauth2_http/javatests/com/google/auth/oauth2/ServiceAccountCredentialsTest.java

Co-authored-by: gemini-code-assist[bot] <176961590+gemini-code-assist[bot]@users.noreply.github.com>

Author: westarle

google-auth-library-java/oauth2_http/javatests/com/google/auth/oauth2/ServiceAccountCredentialsTest.java

@@ -1790,8 +1790,8 @@ private void verifyJwtAccess(Map<String, List<String>> metadata, String expected
     }
     assertEquals(PRIVATE_KEY_ID, signature.getHeader().getKeyId());
 
-    Long iat = (Long) signature.getPayload().get("iat");
-    Long exp = (Long) signature.getPayload().get("exp");
+    Long iat = signature.getPayload().getIssuedAtTimeSeconds();
+    Long exp = signature.getPayload().getExpirationTimeSeconds();
     assertNotNull(iat);
     assertNotNull(exp);
     assertEquals(3600L, exp - iat);