[Task] Add upgrade notes for version 1.3 (#256)

* add upgrade notes

* add upgrade notes

Author: lukmzig

doc/01_Installation/02_Upgrade.md

@@ -2,6 +2,22 @@
 
 Following steps are necessary during updating to newer versions.
 
+## Upgrade to 1.3.0
+- [Indexing] Added support for Elasticsearch in parallel to Opensearch. Opensearch remains the default search technology. If you are using Elasticsearch, you need to update your symfony configuration as follows:
+```yml 
+pimcore_generic_data_index:
+  index_service:
+    client_params:
+      client_name: default
+      client_type: 'elasticsearch'
+```
+- [Indexing] Introduced new service alias `generic-data-index.search-client`. This will replace deprecated alias `generic-data-index.opensearch-client` which will be removed in the next major version. 
+The new service alias can be used to inject the search client into your services. This search client is an instance of `Pimcore\SearchClient\SearchClientInterface` which is a common interface for OpenSearch and Elasticsearch clients.
+- Classes under OpenSearch namespaces are now deprecated and will be removed in the next major version. Please use the classes under the DefaultSearch namespace instead.
+- Execute the following command to reindex all elements to be able to use all new features or when switching between OpenSearch and Elasticsearch:
+
+  ```bin/console generic-data-index:update:index```
+
 ## Upgrade to 1.1.0
 - Execute the following command to reindex all elements to be able to use all new features: