Merged ISpanContext into ITraceContext

[jamescrosswell]

This interface contained no properties or methods... and only impleneted ITraceContext so is entirelyreplacable by ITraceContext.

Migration Notes

Methods and properties now available on ITraceContext (although likely soon to be merged into ISpanData - so we'll probably tell users to look for the methods and properties on ISpanData instead).

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## Unreleased

- Merged ISpanContext into ITraceContext ([#2668](https://github.com/getsentry/sentry-dotnet/pull/2668))

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[bruno-garcia]

Just a note on release notes/migration guide.

[bruno-garcia]

Does this affect the end user in any way? Doesn't seem so, but worth being clear here.

At the end of the day we'll need a migration guide so users know what steps users need to take in order to upgrade the SDK. So when we say a type was removed, they might expect: "Ok what do I need to do to replace it?". In this case it's possibly just "This type had no documented use and if somehow you used it, it can be replaced with ITraceContext" or something like that?

[jamescrosswell]

OK, I'll try to paraphrase that in the changelog for now... and put a bit more detail in the PR notes that we can use when preparing the migration guide proper.