Removal of deprecated NATS Streaming Server (aka Stan)

### Proposal

https://github.com/kedacore/keda/pull/6363, https://nats-io.gitbook.io/legacy-nats-docs/nats-streaming-server-aka-stan

To be removed in 2.19 KEDA release

### Use-Case

_No response_

### Is this a feature you are interested in implementing yourself?

Yes

### Anything else?

_No response_