[submodule] Update submodule sonic-gnmi to the latest HEAD automatically (#15504)

#### Why I did it
src/sonic-gnmi
```
* a600dc9 - (HEAD -> master, origin/master, origin/HEAD) Fix threading issues in Event Client (#121) (9 hours ago) [Zain Budhwani]
```
#### How I did it
#### How to verify it
#### Description for the changelog
This commit is contained in:
mssonicbld 2023-06-16 16:29:27 +08:00 committed by GitHub
parent 0d10c7cbd9
commit c6d242180b
No known key found for this signature in database
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@ -1 +1 @@
Subproject commit 37ce38b01718447aa63de06cc1ebdcb3f1cda160
Subproject commit a600dc954ca253b2acb4dadb52c85ce44e07ff8e