MM-53962: Adding ES8 dependency (#24399)

* Adding ES8 dependency

```release-note
NONE
```


Co-authored-by: Mattermost Build <build@mattermost.com>

server/public/model/config.go

@@ -205,6 +205,8 @@ const (
 	ElasticsearchSettingsDefaultLiveIndexingBatchSize       = 1
 	ElasticsearchSettingsDefaultRequestTimeoutSeconds       = 30
 	ElasticsearchSettingsDefaultBatchSize                   = 10000
+	ElasticsearchSettingsESBackend                          = "elasticsearch"
+	ElasticsearchSettingsOSBackend                          = "opensearch"
 
 	BleveSettingsDefaultIndexDir  = ""
 	BleveSettingsDefaultBatchSize = 10000
@@ -2763,6 +2765,7 @@ func (s *NativeAppSettings) SetDefaults() {
 
 type ElasticsearchSettings struct {
 	ConnectionURL                 *string `access:"environment_elasticsearch,write_restrictable,cloud_restrictable"`
+	Backend                       *string `access:"environment_elasticsearch,write_restrictable,cloud_restrictable"`
 	Username                      *string `access:"environment_elasticsearch,write_restrictable,cloud_restrictable"`
 	Password                      *string `access:"environment_elasticsearch,write_restrictable,cloud_restrictable"`
 	EnableIndexing                *bool   `access:"environment_elasticsearch,write_restrictable,cloud_restrictable"`
@@ -2795,6 +2798,10 @@ func (s *ElasticsearchSettings) SetDefaults() {
 		s.ConnectionURL = NewString(ElasticsearchSettingsDefaultConnectionURL)
 	}
 
+	if s.Backend == nil {
+		s.Backend = NewString(ElasticsearchSettingsESBackend)
+	}
+
 	if s.Username == nil {
 		s.Username = NewString(ElasticsearchSettingsDefaultUsername)
 	}
@@ -4173,6 +4180,10 @@ func (s *ElasticsearchSettings) isValid() *AppError {
 		}
 	}
 
+	if *s.Backend != ElasticsearchSettingsOSBackend && *s.Backend != ElasticsearchSettingsESBackend {
+		return NewAppError("Config.IsValid", "model.config.is_valid.elastic_search.invalid_backend.app_error", nil, "", http.StatusBadRequest)
+	}
+
 	return nil
 }