[submodule] Update submodule sonic-gnmi to the latest HEAD automatically (#15991)

#### Why I did it
src/sonic-gnmi
```
* 2c8e4ab - (HEAD -> master, origin/master, origin/HEAD) Support proto encoding (#140) (22 hours ago) [ganglv]
```
#### How I did it
#### How to verify it
#### Description for the changelog
This commit is contained in:
mssonicbld 2023-07-29 16:32:27 +08:00 committed by GitHub
parent 351162a727
commit 2a69c6c9a1
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@ -1 +1 @@
Subproject commit fb338d50aad1e78d2b675000c916c5b5fdecfc0e
Subproject commit 2c8e4ab783d10b7433325653c1c90ad2b7f980b0