[sonic-utilities] advance submodule head to latest (#2842)

Submodule src/sonic-utilities 6ee0aea..b531934:
  > [db migrator] Introduce the DB migration infrastructure (#519)
  > Skip INTERFACE entries w/o prefix (#477)
  > Bring queue storm status to 'pfcwd show stats' (#500)
  > Align PSU DB count field with the schema Spec. (#509)
  > [scripts] remove duplicate script copying for nbrshow (#517)
  > If fast-reboot-dump gives an error, don't continue with fast-reboot (#515)
  > load_minigraph: restart hostcfgd (#511)
  > [fast/warm reboot] add some sanity check before warm reboot (#510)
  > show BPS, PPS, UTIL rates w/o previous clear (#508)
  > In sync with our latest change, where we default failthrough to be False. (#507)
  > Add warm-boot feature processing for wedge100bf_32x/65x platforms (#485)
  > [generate_dump] system dump improvements (#503)
  > [neighbor advertiser] convert int to string before concatenating (#505)

Signed-off-by: Ying Xie <ying.xie@microsoft.com>