Switch to APPLICATION_JSON

Signed-off-by: Craig Perkins <[email protected]>

src/test/kotlin/org/opensearch/indexmanagement/indexstatemanagement/IndexStateManagementRestTestCase.kt

@@ -457,7 +457,7 @@ abstract class IndexStateManagementRestTestCase : IndexManagementRestTestCase()
                 StringEntity(
                     "{\"doc\":{\"managed_index\":{\"schedule\":{\"interval\":{\"start_time\":" +
                         "\"$startTimeMillis\"}}}}}",
-                    ContentType.APPLICATION_JSON,
+                    APPLICATION_JSON,
                 ),
             )