Fix: SocketException with GetNetworkTime on cell

[nickfourtimes]

During testing, we found that running Time.GetNetworkTime on an iPhone XR (iOS 16.5.1c), on cellular data, caused a SocketException. After thorough investigation with Unity support, it was determined that ineligible (IPv6) addresses were being returned via Dns.GetHostEntryAsync; the present change addresses this.

[nickfourtimes]

To provide more detail: I'd noticed the SocketException while debugging an issue with Time.GetNetworkTime, and escalated the issue with our contacts at Unity support. After some thorough investigation, these were their findings:

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After a lot of debugging I found the problem, it turns out that over cellular data Dns.GetHostEntryAsync includes IPv6 in the results array, while over wifi it only lists IPv4 ones, so it ended up attempting to create an endpoint with an IPv6 address.

When converting the endpoint to string I got the following values:

• Over wifi: 17.253.12.125:123 (one of the IPv4 of time.apple.com)
• Over cellular data: [2620:149:a18:4000::1f2]:123 (one of the IPv6 of time.apple.com)

In its documentation I can see Dns.GetHostEntryAsync filters IPv6 addresses when the requesting device doesn't have IPv6 installed. I suspect some servers from the cellular data chain connection act as proxy affecting the dns lookup; I'm not entirely sure that's the reason, but I managed to solve the issue by ignoring the IPv6 addresses.

The original code takes the object 0 from the IP results array from Dns.GetHostEntryAsync. I did some code for iterating over the array and use the first IPv4 address, now I can get a valid IP for creating a valid endpoint, thus opening the socket correctly:

[Listing from Apple.Core/Runtime/Time.cs, as included in this pull request]