[sonic-snmpagent] Update sonic-snmpagent submodule (#11467)

151638b Change diff coverage threshold to 80% (#261)
a45f623 Replace swsssdk with swsscommon (#263) 
3bd4a23 Remove python .whl package dependency to swsssdk (#265) 

Co-authored-by: liuh-80 <azureuser@liuh-dev-vm-02.5fg3zjdzj2xezlx1yazx5oxkzd.hx.internal.cloudapp.net>