Case insensitive when set doc engine (#3954)

### What problem does this PR solve?

DOC_ENGINE="INFINITY" or "Infinity" or "Elasticsearch" also works

### Type of change

- [x] Refactoring

Signed-off-by: Jin Hai <haijin.chn@gmail.com>

api/settings.py

 
     global DOC_ENGINE, docStoreConn, retrievaler, kg_retrievaler
     DOC_ENGINE = os.environ.get('DOC_ENGINE', "elasticsearch")
-    if DOC_ENGINE == "elasticsearch":
+    lower_case_doc_engine = DOC_ENGINE.lower()
+    if lower_case_doc_engine == "elasticsearch":
         docStoreConn = rag.utils.es_conn.ESConnection()
-    elif DOC_ENGINE == "infinity":
+    elif lower_case_doc_engine == "infinity":
         docStoreConn = rag.utils.infinity_conn.InfinityConnection()
     else:
         raise Exception(f"Not supported doc engine: {DOC_ENGINE}")