[submodule]: Update submodule sonic-linkmgrd (sonic-net#9421)

6c6151b Fix unstable unit tests (state change handler wasn't invoked) (zhenggen-xu#8)
2f7dc0a support code diff coverage (zhenggen-xu#5)
83f0002 Force mux state switch to standby if triggered from Cli (zhenggen-xu#6)

signed-off-by: Jing Zhang [email protected]