update sonic-snmpagent pointer (#12706)

#### Why I did it

Update sonic-snmpagent ubmodule pointer to include the following:

* bf6cd4c Remove error logging on "failed in fdb_vlanmac" ([#272](sonic-net/sonic-snmpagent#272))
* 05f2a28 Remove LOGLEVEL DB since is no longer used ([#271](sonic-net/sonic-snmpagent#271))
* 06299e1 Don't cache the vlan-id if it is not valid from DB ([#273](sonic-net/sonic-snmpagent#273))
* 8a515f2 Install libyang to azure pipeline ([#268](sonic-net/sonic-snmpagent#268))

#### How I did it

Advance sonic-snmpagent pointer