[sonic-swss][202012] submodule update (#8011)

Advance submodule head for sonic-swss on 202012

bb383be2 [Dynamic Buffer Calc][Mellanox] Bug fixes and enhancements for the lua plugins for buffer pool calculation and headroom checking (sonic-net/sonic-swss#1781)
f949dfe9 [Dynamic Buffer Calc] Avoid creating lossy PG for admin down ports during initialization (sonic-net/sonic-swss#1776)
def0a914 Fix config prompt question issue (sonic-net/sonic-swss#1799)
21f97506 [ci]: Merge azure pipelines from master to 202012 branch (sonic-net/sonic-swss#1764)
a83a2a42 [vstest]: add dvs_route fixture
849bdf9c [Mux] Add support for mux metrics to State DB (sonic-net/sonic-swss#1757)
386de717 [qosorch] Dot1p map list initialization fix (sonic-net/sonic-swss#1746)
f99abdca [sub intf] Port object reference count update (sonic-net/sonic-swss#1712)
4a00042d [vstest/nhg]: use dvs_route fixture to make test_nhg more robust

Signed-off-by: Stephen Sun <stephens@nvidia.com>