refactor: cleanup DNS resolver init

[lidel]

This cleanup adapts to boxo's switch from hardcoded DNS resolvers to defaults provided by boxo/autoconf

• simplify DNS resolver initialization (call gateway.NewDNSResolver directly)
• add regression test verifying autoconf DNS resolvers are expanded correctly
• adapt to bitswap RebroadcastDelay API change (time.Duration)
• update boxo to ipfs/boxo@5ffe139 (needs to be switched to boxo release once https://github.com/ipfs/boxo/pull/1044 ships)

Related

• Depends on https://github.com/ipfs/boxo/pull/1044