Workaround a C++/CLI bug involving DIMs

[tannergooding]

As per the title there is currently a C++/CLI bug involving implementing an instance DIM on a derived interface. This causes C++/CLI to error when using any type that implements such an interface, which in the case of System.Numerics.INumberBase<TSelf> includes all the primitive types (such as Int32 or Single).

There were a few possible changes that would temporarily resolve this issue...

1. This change, where we stop implementing the interface but keep exposing the general functionality on the interface until we can revert.
2. We could have updated IUtf8SpanFormattable to have a DIM where it always throws PlatformNotSupportedException, but this could lead to unexpected behavior for consumers of the API
3. We could have updated IUtf8SpanFormattable to have a DIM where it always returns false, but this risks users naively doing the wrong thing and trying to grow the buffer and call the API repeatedly as that's how the other TryFormat APIs work
4. We could have updated IUtf8SpanFormattable to inherit from ISpanFormattable and have a DIM where it defers to that implementation

Of these options, 1 or 4 are the least risky.

4 would end up changing the shipping API surface and would not require a reversion later. It has the downside in that it requires everyone implementing IUtf8SpanFormattable to also support ISpanFormattable (that is UTF-16). This puts more restriction on the API surface overall. That being said, it is the better option if we believe that the C++/CLI fix will not make the .NET 8 release as avoids us introducing a diamond problem in the future as the DIM is provided in the same release that the relevant interface and API are defined.

1 (this change) will require us to revert this PR once the C++/CLI fix goes in. Not reverting this workaround will result in INumberBase<TSelf> not being able to implement IUtf8SpanFormattable without risking the introduction of a diamond problem, since other types/interfaces could implement IUtf8SpanFormattable with a DIM before the .NET 9 release.

[dotnet-issue-labeler]

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Issue Details

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Author:	tannergooding
Assignees:	-
Labels:	area-System.Numerics, new-api-needs-documentation
Milestone:	-

[jeffhandley]

This temporary workaround looks good to me, with the understanding that we'll revert it once the C++/CLI team applies their fix and we take that update into dotnet/wpf.

[jeffhandley]

/backport to release/8.0-preview7

[github-actions]

Started backporting to release/8.0-preview7: https://github.com/dotnet/runtime/actions/runs/5614063183