feat(core): Deprecate span.startChild()

[mydea]

We still have quite a lot of these ourselves, but we can refactor them over time during the v8 preparation.

But with this, I think we have the most important deprecations mostly done!

[Lms24]

LGTM! So to confirm, the intended replacement strategy is to no longer start a child for a specific span but use startSpan and therefore start a child for whatever span is currently active on the scope?

[mydea]

LGTM! So to confirm, the intended replacement strategy is to no longer start a child for a specific span but use startSpan and therefore start a child for whatever span is currently active on the scope?

Yes! We need to go through all our usage. We also need to deprecate getActiveTransaction() which we use a lot there. This should just be auto-handled by startSpan()!