network/ndisc: use router lifetime as one for redirect route

network/ndisc: use router lifetime as one for redirect route

Previously, we did not set lifetime for redirect route, and redirect
routes were removed only when received a RA from the target address.
Thus, routes that redirect on-link addresses were never removed.

RFCs mention nothing about the lifetime of redirection. But the previous
implementation does not pass the IPv6 Core Conformance Tests.

This makes
- remember all received RAs and manage them by the sender address
  (previously, remembered only one with the highest preference),
- then use the router lifetime as one for redirect route,
- remove redirect route also when the router corresponds to the sender
  address is dropped (previously, considered only target address).

Note, even if we recieve a new RA, we do not update existing redirect
routes. The lifetime of the redirect route is updated only when a new
Redirect message is received.

Closes #32527.