[v8] Remove startTransaction API #9925

In v8, we have to get rid of startTransaction, in favor of the (already existing) startSpan(), startInactiveSpan() and startManualSpan() APIs. Since this is not compatible with OTEL, we have to get rid of it for Node, and for consistency & keeping us flexible for an only-spans future, we should get rid of it in Browser as well.

### Tasks
- [x] Refactor all existing usage of `startTransaction` in the SDK
- [x] Deprecate `startTransaction`
- [x] Remove `startTransaction` in v8
- [x] Refactor `IdleTransaction`