fix:remove collection schema cache after dropCollection (#1550)

Signed-off-by: uncle <772817297@qq.com>
Co-authored-by: uncle <772817297@qq.com>

sdk-core/src/main/java/io/milvus/v2/client/MilvusClientV2.java

@@ -320,6 +320,7 @@ public class MilvusClientV2 {
      */
     public void dropCollection(DropCollectionReq request) {
         rpcUtils.retry(()-> collectionService.dropCollection(this.getRpcStub(), request));
+        vectorService.removeCollectionCache(request.getDatabaseName(), request.getCollectionName());
     }
     /**
      * Alter a collection in Milvus.

sdk-core/src/main/java/io/milvus/v2/service/vector/VectorService.java

@@ -99,7 +99,7 @@ public class VectorService extends BaseService {
         }
     }
 
-    private void removeCollectionCache(String databaseName, String collectionName) {
+    public void removeCollectionCache(String databaseName, String collectionName) {
         String key = GTsDict.CombineCollectionName(actualDbName(databaseName), collectionName);
         cacheCollectionInfo.remove(key);
     }