Add app arch team as owner of datemath package (#66880)

Co-authored-by: Elastic Machine <elasticmachine@users.noreply.github.com>

.github/CODEOWNERS

@@ -37,6 +37,7 @@
 /examples/url_generators_examples/ @elastic/kibana-app-arch
 /examples/url_generators_explorer/ @elastic/kibana-app-arch
 /packages/kbn-interpreter/ @elastic/kibana-app-arch
+/packages/elastic-datemath/ @elastic/kibana-app-arch
 /src/legacy/core_plugins/embeddable_api/ @elastic/kibana-app-arch
 /src/legacy/core_plugins/interpreter/ @elastic/kibana-app-arch
 /src/legacy/core_plugins/kibana_react/ @elastic/kibana-app-arch