Back-port fix for producing correct UTC DateTime values by default, to ASP .NET 3.1.x

[augustoproiete]

This is a follow-up from issue #11584 which was fixed by PR #24893 but seems it will be available in only in .NET 5.

As it stands today and described in more detail in #11584, sending a UTC time string such as 2020-11-07T02:30:04.0576719Z to an ASP .NET Core 3.1 API converts it to a DateTime with DateTimeKind.Local instead of DateTimeKind.Utc causing the API to think a conversion to UTC is necessary, leading to bad data and bugs in applications.

I completely understand the policy of not back-porting non-critical bug fixes to earlier releases which makes sense, but I am advocating that this is indeed a very critical bug that should be back-ported to (at least) ASP .NET 3.1.x. given that being able to correctly parse a UTC time string is fundamental to most (if not all) applications 😅.

If back-porting is not an option, then properly documenting it on docs.microsoft.com with a code sample/template would be the next best thing.

/cc @pranavkm

[captainsafia]

@augustoproiete While we won't be back-porting this bug-fix, you can use the code in #11584 (comment) as a reference for how to implement a fix in existing 3.1 applications.

[ghost]

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