[sonic-utilities] submodule update (#7546)

Includes below commits

9a88cb6 2021-05-06 | [sonic_installer] dont fail package migration (#1591) [Stepan Blyshchak]
615e531 2021-05-05 | [show][config] Add new snmp commands (#1347) [Travis Van Duyn]
fff4051 2021-05-05 | Fixing serial number read to get from DB if it is populated (#1580) [Sudharsan Dhamal Gopalarathnam]
be974bf 2021-05-05 | [neighbor_advertiser] Use existing tunnel if present for creating tunnel mappings (#1589) [Sumukha Tumkur Vani]
9492eab 2021-05-04 | Use swsscommon instead of swsssdk (#1510) [Andriy Yurkiv]
0f4988b 2021-05-04 | Add pg-drop script to sonic filesystem (#1583) [Andriy Yurkiv]
cbe2159 2021-05-04 | [vnet] Add "vnet_route_check" script (#1300) [Volodymyr Samotiy]
9120766 2021-05-03 | Relax the install_requires, no need to exact version as long as there are no broken changes with future versions (#1530) [Qi Luo]
2e09b22 2021-05-03 | Handle the new db version which mellanox_buffer_migrator isn't interested (#1566) [Stephen Sun]

Author: qiluo-msft