[submodule] Update submodule sonic-gnmi to the latest HEAD automatically (#24806)

#### Why I did it
src/sonic-gnmi
```
* c7a1914 - (HEAD -> master, origin/master, origin/HEAD) use 1ES agent pool to provide more disk space (#551) (4 days ago) [yijingyan2]
```
#### How I did it
#### How to verify it
#### Description for the changelog

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

Author: mssonicbld