Revert "Set SocketsHttpHandler's default connect timeout to 15s (#66607)"

[CarnaViire]

This reverts commit 436b97c.

The change proved to be problematic as-is, because

1. There is a high chance that the user code has a retry logic that does retry when SocketException is caught but does not retry in case of OperationCanceledException. This can lead to a spike of (unretried) exceptions which weren't there before. Example:
   https://github.com/NuGet/NuGet.Client/blob/93bb3921490e9d499be1fb48d6cb9dcaff734db1/src/NuGet.Core/NuGet.Protocol/HttpSource/HttpRetryHandler.cs#L212-L263

2. The workaround with setting the ConnectTimeout back to infinity may not be available -- if HttpClientHandler is used instead of SocketsHttpHandler (e.g. due to reusing code between .NET Framework and .NET Core/.NET 5+), or if the code that executes the requests is hidden inside a library.

I will reopen #66297 for further considerations.

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Issue Details

---

This reverts commit 436b97c.

The change proved to be problematic as-is, because

1. There is a high chance that the user code has a retry logic that does retry when SocketException is caught but does not retry in case of OperationCanceledException. This can lead to a spike of (unretried) exceptions which weren't there before. Example:
   https://github.com/NuGet/NuGet.Client/blob/93bb3921490e9d499be1fb48d6cb9dcaff734db1/src/NuGet.Core/NuGet.Protocol/HttpSource/HttpRetryHandler.cs#L212-L263

2. The workaround with setting the ConnectTimeout back to infinity may not be available -- if HttpClientHandler is used instead of SocketsHttpHandler (e.g. due to reusing code between .NET Framework and .NET Core/.NET 5+), or if the code that executes the requests is hidden inside a library.

I will reopen #66297 for further considerations.

Author:	CarnaViire
Assignees:	-
Labels:	area-System.Net.Http
Milestone:	-