Add bwc assertions until the backports are completed.

Author: gmarouli

qa/rolling-upgrade/src/javaRestTest/java/org/elasticsearch/upgrades/SourceModeRollingUpgradeIT.java

@@ -85,8 +85,13 @@ private void assertDeprecationWarningForTemplate(String templateName) throws IOE
         assertThat(response.containsKey("templates"), equalTo(true));
         Map<?, ?> issuesByTemplate = (Map<?, ?>) response.get("templates");
         assertThat(issuesByTemplate.containsKey(templateName), equalTo(true));
-        var templateIssues = (List<?>) issuesByTemplate.get(templateName);
-        assertThat(((Map<?, ?>) templateIssues.getFirst()).get("message"), equalTo(SourceFieldMapper.DEPRECATION_WARNING_TITLE));
-        assertThat(((Map<?, ?>) templateIssues.getFirst()).get("details"), equalTo(SourceFieldMapper.DEPRECATION_WARNING));
+        var templateIssue = (Map<?, ?>) ((List<?>) issuesByTemplate.get(templateName)).getFirst();
+        // Bwc compatible logic until backports are complete.
+        if (templateIssue.containsKey("details")) {
+            assertThat(templateIssue.get("message"), equalTo(SourceFieldMapper.DEPRECATION_WARNING_TITLE));
+            assertThat(templateIssue.get("details"), equalTo(SourceFieldMapper.DEPRECATION_WARNING));
+        } else {
+            assertThat(templateIssue.get("message"), equalTo(SourceFieldMapper.DEPRECATION_WARNING));
+        }
     }
 }