Upgrade gnxi to support dash (#16498)

### Why I did it
Need new gnmi client for dash test.

### How I did it
I have updated gnxi repo, and this PR is used to get latest change.

#### How to verify it
Run end2end test for DASH.

Author: ganglyu

dockers/docker-ptf/Dockerfile.j2

@@ -209,7 +209,7 @@ RUN mkdir -p /var/log/supervisor
 # Install Python-based GNMI client
 RUN git clone https://github.com/lguohan/gnxi.git \
     && cd gnxi \
-    && git checkout b85e4ab \
+    && git checkout 3adf8b9 \
     && cd gnmi_cli_py \
     && pip install -r requirements.txt