deps: Update libp2p to v0.28

[Wondertan]

We already tried to update #2127 and then had to revert #2136. The reason is hidden in libp2p/go-libp2p#1574, where responsibility for sending events is now moved to the Network/Swarm from Host. However, mocknet was not updated to send notifications, causing our tests to fail.

I submitted the fix libp2p/go-libp2p#2287 and waiting for the reviews there. Once it lands, we should be able to upgrade. Alternatively, libp2p/go-libp2p#2293 might be introduced, which will likely require some migration