[submodule] Update sonic-snmpagent submodule (#7859)

Update sonic-snmpagent submodule to pick up new commits:

21d7d97 2021-07-12 Fix: SonicV2Connector behavior change: get_all will return empty dict if (#226)
0813b42 2021-07-12 Entries under .1.3.6.1.2.1.31.1.1.1.18 OID should return the "description" field of PORT_TABLE entries in APPL_DB or CONFIG_DB. (#224)
7a78703 2021-07-08 Install dotnet core to fix python gcov warning for code covery color bar showing (#215)
e0f36a5 2021-06-30 [multi-asic]: Udpate to use SonicDBConfig from swsscommon (#219)
266bd15 2021-06-10 Restored snmp vlan support per RFC1213 and added the missing support for RFC2863 (#218)