[submodule] Update submodule sonic-gnmi to the latest HEAD automatica…

…lly (#16799)

#### Why I did it
src/sonic-gnmi
```
* df4d49f - (HEAD -> 202305, origin/202305) Install necessary debs instead of entire artifact in azp (#137) (12 hours ago) [Zain Budhwani]
```
#### How I did it
#### How to verify it
#### Description for the changelog