Replace/Simplify MinimumAgeRequirement in Authz Policies

As per https://github.com/dotnet/AspNetCore.Docs/pull/23658#discussion_r737918216, we'd like to replace/simplify `MinimumAgeRequirement` with something that involves less date logic.

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#### Document Details

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* ID: b1141d92-adeb-6841-229f-914c808f9a2b
* Version Independent ID: 7089b376-949f-c1c6-c37a-5d8cc7ecf104
* Content: [Policy-based authorization in ASP.NET Core](https://docs.microsoft.com/en-us/aspnet/core/security/authorization/policies?view=aspnetcore-6.0)
* Content Source: [aspnetcore/security/authorization/policies.md](https://github.com/dotnet/AspNetCore.Docs/blob/main/aspnetcore/security/authorization/policies.md)
* Product: **aspnet-core**
* Technology: **aspnetcore-security**
* GitHub Login: @Rick-Anderson
* Microsoft Alias: **riande**