[submodule] Update submodule sonic-gnmi to the latest HEAD automatically (sonic-net#23836)

#### Why I did it
src/sonic-gnmi
```
* 776f968 - (HEAD -> master, origin/master, origin/HEAD) Implement `show chassis module status` (sonic-net#484) (59 minutes ago) [jingwenxie]
```
#### How I did it
#### How to verify it
#### Description for the changelog

Signed-off-by: Feng Pan <[email protected]>

Author: mssonicbld