[submodule] Update submodule linkmgrd to the latest HEAD automatically (#19402)

#### Why I did it
src/linkmgrd
```
* 9768268 - (HEAD -> master, origin/master, origin/HEAD) Fix default route race condition UT (#258) (22 hours ago) [Longxiang Lyu]
```
#### How I did it
#### How to verify it
#### Description for the changelog

Author: mssonicbld