[Obs AI Assistant] Use sparse_vector instead of text_expansion (#202891)

Closes #202313

The [text expansion
query](https://www.elastic.co/guide/en/elasticsearch/reference/current/query-dsl-text-expansion-query.html)
was deprecated in 8.15 and is being replaced by [sparse vector
query](https://www.elastic.co/guide/en/elasticsearch/reference/current/query-dsl-sparse-vector-query.html).

x-pack/platform/plugins/shared/observability_solution/observability_ai_assistant/server/service/knowledge_base_service/recall_from_search_connectors.ts

@@ -155,12 +155,11 @@ async function recallFromLegacyConnectors({
         bool: {
           should: [
             {
-              text_expansion: {
-                [vectorField]: {
-                  model_text: text,
-                  model_id: modelId,
-                  boost,
-                },
+              sparse_vector: {
+                field: vectorField,
+                query: text,
+                inference_id: modelId,
+                boost,
               },
             },
           ],