[submodule] Update submodule for sonic-swss-common (#7358)

804c9f9 [sonic-swss-common] Add a string utilities function to join items in a container (#463)
1e924ac Fix: ConfigDBConnector call super init with proper parameter name (#470)
70e02c8 [azp] Purge swss before installing the newly built deb package (#472)
15c0f72 [swig] translate SonicV2Connector::keys return type from C++ vector to Python list (#468)

Author: Junchao-Mellanox