[v8] Remaining things to deprecate before v8

[mydea]

These things still need to be deprecated (and/or a replacement written) before v8:

### Tasks
- [ ] `getCurrentHub()` - @deprecated & migration docs
- [ ] `startTransaction()` - @deprecated & migration docs
- [ ] All methods and properties on `Hub` interface
- [ ] https://github.com/getsentry/sentry-javascript/issues/10184
- [ ] All methods and properties `Transaction` that do not exist on OTEL spans
- [ ] lastEventId - make global method/variable
- [ ] `scope.setClient()` & update `Sentry.getClient()` to use scope clients (if set)
- [ ] `client.init()` instead of `client.setupIntegrations()` as public API
- [ ] makeMain()

[Lms24]

Some thoughts (should be formalized into tasks) around sessions:

• let's move the node-experimental model to core
  • add global startSession/endSession function
  • client.captureSession instead of hub.captureSession
  • add global captureSession (necessary because hub.capturesession does more than client.capturesession)

[AbhiPrasad]

Closing this as I think we've gotten through this list!