[Obs AI Assistant] Remove semantic text feature flag (#217750)

Closes https://github.com/elastic/kibana/issues/206827

## Summary

Semantic text was updated to a new format with
https://github.com/elastic/dev/issues/2936 to act like a normal text
field. This was under a feature flag, therefore we had to enforce the
new format by setting the feature flag
`index.mapping.semantic_text.use_legacy_format` to `false`.

With the merge of https://github.com/elastic/elasticsearch/pull/120813
the new semantic format is now the default behaviour. Therefore, this PR
removes the feature flag in our code.

## Querying the KB after removing the feature flag


https://github.com/user-attachments/assets/8f91bd5a-033a-47ec-9bed-3e2bdfe85627


### Checklist

- [x] The PR description includes the appropriate Release Notes section,
and the correct `release_note:*` label is applied per the
[guidelines](https://www.elastic.co/guide/en/kibana/master/contributing.html#kibana-release-notes-process)

x-pack/platform/plugins/shared/observability_ai_assistant/server/service/startup_migrations/create_or_update_index_assets.ts

@@ -106,7 +106,6 @@ export async function createOrUpdateIndexAssets({
           number_of_shards: 1,
           auto_expand_replicas: '0-1',
           hidden: true,
-          'index.mapping.semantic_text.use_legacy_format': false,
         },
       },
     });

x-pack/solutions/observability/plugins/observability_ai_assistant_app/scripts/evaluation/scenarios/elasticsearch/index.spec.ts

@@ -48,9 +48,6 @@ describe('Elasticsearch functions', () => {
               },
             },
           },
-          settings: {
-            'index.mapping.semantic_text.use_legacy_format': false,
-          },
         });
 
         await esClient.index({

x-pack/solutions/observability/plugins/observability_ai_assistant_app/scripts/evaluation/scenarios/esql/index.spec.ts

@@ -72,9 +72,6 @@ describe('ES|QL query generation', () => {
               },
             },
           },
-          settings: {
-            'index.mapping.semantic_text.use_legacy_format': false,
-          },
         });
 
         await esClient.index({
@@ -128,9 +125,6 @@ describe('ES|QL query generation', () => {
               },
             },
           },
-          settings: {
-            'index.mapping.semantic_text.use_legacy_format': false,
-          },
         });
 
         await esClient.index({