[submodule] Update submodule sonic-gnmi to the latest HEAD automatically (sonic-net#20455)

#### Why I did it
src/sonic-gnmi
```
* 2b7f8a1 - (HEAD -> master, origin/master, origin/HEAD) Add mgmt VRF support. (sonic-net#290) (58 minutes ago) [Hua Liu]
* 646b1f5 - Increase dbus timeout for gcu (sonic-net#295) (3 days ago) [ganglv]
```
#### How I did it
#### How to verify it
#### Description for the changelog

Author: mssonicbld