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

#19979)

#### Why I did it
src/linkmgrd
```
* 26dbdc7 - (HEAD -> 202405, origin/202405) [active-standby] Fix default route handler race condition (#254) (10 hours ago) [Longxiang Lyu]
```
#### How I did it
#### How to verify it
#### Description for the changelog