Remove Obsolete Extra members

### Description

`SentrySpan`, `SentryTransaction` and `TransactionTracer` all have `Extra` members like:
https://github.com/getsentry/sentry-dotnet/blob/748fcfe7a12460e5b6433265de048e1cb6a46b2e/src/Sentry/SentrySpan.cs#L82-L84
https://github.com/getsentry/sentry-dotnet/blob/748fcfe7a12460e5b6433265de048e1cb6a46b2e/src/Sentry/SentrySpan.cs#L86-L88

Although we do probably want to get rid of these, as believe we also want to get rid of the `Data` members that we're recommending people switch to... since I think all of these will be replaced by Attributes at some stage right?

Possibly we delay replacing these until the new Attributes are available and, at that point, we could ship an Analyzer and a Code fix to help people migrate (since Extras may be used quite widely).

## See Also

- https://github.com/getsentry/sentry-dotnet/pull/4619#discussion_r2432324349