[submodule][202506] Update submodule sonic-swss to the latest HEAD automatically (#1865)

#### Why I did it
src/sonic-swss
```
* 6ffc8bc7 - (HEAD -> 202506, origin/202506) [action] [PR:3879] fpmsyncd crashes during execution of sonic-mgmt script vxlan/test_vnet_bgp_route_precedence.py (#176) (9 hours ago) [mssonicbld]
```
#### How I did it
#### How to verify it
#### Description for the changelog

Author: mssonicbld