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

…lly (#18076)

#### Why I did it
src/sonic-gnmi
```
* d56712a - (HEAD -> master, origin/master, origin/HEAD) Update GNMI path schema (#197) (4 days ago) [ganglv]
* 758ec18 - Call flag.Parse() to parse global flags like -logtostderr (#198) (5 days ago) [Zain Budhwani]
* 736e3b4 - Add signal handler to stop gnmi server for when sigterm or sigquit is called (#189) (3 weeks ago) [Zain Budhwani]
* 5b59c57 - Fix sonic string in osversion/build (#190) (4 weeks ago) [Zain Budhwani]
* d8d15c7 - Enable unit tests and code coverage for telemetry.go (#186) (5 weeks ago) [Zain Budhwani]
```
#### How I did it
#### How to verify it
#### Description for the changelog