[202012][swss]: Submodule update

* Include the following commits:
  - a21b160 [202012][orchagent]: Handle duplicate routes in a graceful manner (sonic-net#2666)
  - 1540161 [bfdorch] add default TOS value for BFD packet (sonic-net#2692)
  - 860430c [ci] run apt-get update before apt-get install (sonic-net#2686)

Signed-off-by: Prabhat Aravind <[email protected]>

Author: prabhataravind