[submodule] Update submodule sonic-gnmi to the latest HEAD automatically (#19129)

#### Why I did it
src/sonic-gnmi
```
* 50817e6 - (HEAD -> master, origin/master, origin/HEAD) Add KillProcess gNOI API (#213) (3 days ago) [isabelmsft]
* 3ea4fe5 - Revert "Account for GLOBAL key in PFC_WD (#178)" (#221) (5 days ago) [Zain Budhwani]
```
#### How I did it
#### How to verify it
#### Description for the changelog

Author: mssonicbld