update sonic-snmpagent pointer (sonic-net#12706)

#### Why I did it

Update sonic-snmpagent ubmodule pointer to include the following:

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

#### How I did it

Advance sonic-snmpagent pointer