ot::BorderRouter::RoutingManager::kMaxPublishedPrefixes variable

This constant specifies the maximum number of route prefixes that may be published by `RoutingManager` in Thread Network Data. This is used by `NetworkData::Publisher` to reserve entries for use by `RoutingManager`. The number of published entries accounts for: - Route prefix `fc00::/7` or `::/0` - One entry for NAT64 published prefix. - One extra entry for transitions.