Consider deprecating Base64Url

[damianh]

@joegoldman2 wrote:

.NET 9 adds support for Base64 Url (dotnet/runtime#1658). It's also compatible with .NET Standard 2.0. The class Base64Url should probably be deprecated in favor of the built-in support in the BCL.

IdentityModel/IdentityModel#580

[damianh]

If we were to adopt this we would need to add a net9.0 target to IdentityModel and have some conditional package references.
Ref dotnet/runtime#1658 (comment) .

Resulting package references:

• netstandard2.0 : System.Text.Json.8.0.5 and System.BCL.Memory.9.0.0
• net8.0: System.BCL.Memory.9.0.0
• net9.0: none