[submodule] Update submodule sonic-gnmi to the latest HEAD automatically (#16228)

#### Why I did it
src/sonic-gnmi
```
* ac94807 - (HEAD -> master, origin/master, origin/HEAD) Support get all with json (edge-core#145) (19 hours ago) [ganglv]
```
#### How I did it
#### How to verify it
#### Description for the changelog

Author: mssonicbld