[submodule] Advance sonic-swss-common pointer (#13556)

Update sonic-swss-common submodule pointer to include the following:
* 6b6842a [NotificationProducer] add pipeline support ([#708](sonic-net/sonic-swss-common#708))
* 2cb5ea0 Increase the netlink buffer size from 3MB to 16MB. ([#739](sonic-net/sonic-swss-common#739))
* dacbdad RedisPipeline ignore flush when call dtor from another thread. ([#736](sonic-net/sonic-swss-common#736))

Signed-off-by: dprital <drorp@nvidia.com>