fix: Elasticsearch: Request Entity Too Large #28117 (#29062)

Fix for gitea putting everything into one request without batching and
sending it to Elasticsearch for indexing as issued in #28117

This issue occured in large repositories while Gitea tries to 
index the code using ElasticSearch.

I've applied necessary changes that takes batch length from below config
(app.ini)
```
[queue.code_indexer]
BATCH_LENGTH=<length_int>
```
and batches all requests to Elasticsearch in chunks as configured in the
above config

modules/indexer/code/elasticsearch/elasticsearch.go

@@ -180,12 +180,18 @@ func (b *Indexer) Index(ctx context.Context, repo *repo_model.Repository, sha st
 	}
 
 	if len(reqs) > 0 {
+		esBatchSize := 50
+
+		for i := 0; i < len(reqs); i += esBatchSize {
 			_, err := b.inner.Client.Bulk().
 				Index(b.inner.VersionedIndexName()).
-			Add(reqs...).
+				Add(reqs[i:min(i+esBatchSize, len(reqs))]...).
 				Do(ctx)
+			if err != nil {
 				return err
 			}
+		}
+	}
 	return nil
 }