litep2p/peerstore: Fix bump last updated time (#4971)

This PR bumps the last time of a reputation update of a peer.
Doing so ensures the peer remains in the peerstore for longer than 1
hour.

Libp2p updates the `last_updated` field as well.

Small summary for the peerstore:
- A: when peers are reported the `last_updated` time is set to current
time (not done before this PR)
- B: peers that were not updated for 1 hour are removed from the
peerstore
- the reputation of the peers is decaying to zero over time
- peers are reported with a reputation change (positive or negative
depending on the behavior)

Because, (A) was not updating the `last_updated` time, we might lose the
reputation of peers that are constantly updated after 1hour because of
(B).

cc @paritytech/networking

Signed-off-by: Alexandru Vasile <[email protected]>