all: use http package to replace http method names (#26535)

rpc/http.go

@@ -209,7 +209,7 @@ func (hc *httpConn) doRequest(ctx context.Context, msg interface{}) (io.ReadClos
 	if err != nil {
 		return nil, err
 	}
-	req, err := http.NewRequestWithContext(ctx, "POST", hc.url, io.NopCloser(bytes.NewReader(body)))
+	req, err := http.NewRequestWithContext(ctx, http.MethodPost, hc.url, io.NopCloser(bytes.NewReader(body)))
 	if err != nil {
 		return nil, err
 	}