[submodule] Update submodule linkmgrd to the latest HEAD automatically (

#19843)

#### Why I did it
src/linkmgrd
```
* b068f02 - (HEAD -> 202305, origin/202305) [active-standby] Fix default route handler race condition (#254) (2 days ago) [Longxiang Lyu]
```
#### How I did it
#### How to verify it
#### Description for the changelog