sonic-utilities submodule update

fff4051 Fixing serial number read to get from DB if it is populated (sonic-net#1580)
be974bf [neighbor_advertiser] Use existing tunnel if present for creating tunnel mappings (sonic-net#1589)
9492eab Use swsscommon instead of swsssdk (sonic-net#1510)
0f4988b Add pg-drop script to sonic filesystem (sonic-net#1583)
cbe2159 [vnet] Add "vnet_route_check" script (sonic-net#1300)
9120766 Relax the install_requires, no need to exact version as long as there are no broken changes with future versions (sonic-net#1530)
2e09b22 Handle the new db version which mellanox_buffer_migrator isn't interested (sonic-net#1566)