Allow nullable lifecycle and data_retention template values

These keys allow using an explicit `null` as the value to "unset" the configuration when merging multiple component templates. As related to the merging tables seen in elastic/elasticsearch#95979 (comment)

Relates also to the discussion in elastic#2049

Author: dakrone

specification/indices/_types/DataLifecycle.ts

@@ -25,7 +25,7 @@ import { ByteSize } from '@_types/common'
  * Data lifecycle denotes that a data stream is managed by DLM and contains the configuration.
  */
 export class DataLifecycle {
-  data_retention?: Duration
+  data_retention?: Duration | null
 }
 
 /**

specification/indices/put_index_template/IndicesPutIndexTemplateRequest.ts

@@ -66,5 +66,5 @@ export class IndexTemplateMapping {
    * @availability stack since=8.8.0 stability=experimental
    * @availability serverless stability=experimental
    */
-  lifecycle?: DataLifecycle
+  lifecycle?: DataLifecycle | null
 }