otlp-collector: Fix elasticsearch.index.prefix name

Otherwise, it's not storing records in the right index.

Refs. swh/infra/sysadm-environment#4955

cluster-configuration/templates/otlp-collector/_collector.tpl

@@ -246,8 +246,8 @@ config:
         value: {{ .Values.clusterName }}
         action: insert
       # for dynamic indexation, the environment is the index prefix
-      - key: index.prefix
-        value: {{ ( print $environment "-" ) }}
+      - key: elasticsearch.index.prefix
+        value: {{ print $environment "-" }}
         action: insert
     attributes/regexp_insert:
       actions: