[roll] Roll fuchsia [netstack] Simplify DHCP address pool structs

This CL makes the following changes to the AddressPool and
ManagedAddresses structs:

1) Remove the `broadcast` and `network_id` members, which were unused in
the DHCP server.

2) Replace the subnet mask Ipv4Address with the equivalent u8
`prefix_len` in the AddressPool struct. The ManagedAddresses struct
uses a SubnetMask type so we can avoid adding the error handling logic
for converting from a u8 => SubnetMask at the point of use. Instead,
we handle that error when converting from AddressPool =>
ManagedAddresses.

Original-Fixed: 73076
Original-Reviewed-on: https://fuchsia-review.googlesource.com/c/fuchsia/+/510758
Original-Revision: 65367dafc03c84a54bf8f9aac11a419d3b3682eb
GitOrigin-RevId: c937abf04eec2324cef07c2e956b97a1a7f5dc6a
Change-Id: I9b7ff81e465fd4fc464eea628df1dc4168e8d9a8