Replace ndjson with jsonl

### 🤔 What's the problem you're trying to solve?

The ndjson spec is defunct (https://github.com/ndjson/ndjson-spec/issues/35#issuecomment-1285673417)

### ✨ What's your proposed solution?

It looks like jsonl is a suitable replacement.

https://jsonlines.org/

* Replace all mentions of ndjson
* Update all file extensions to jsonl
* Create issues for down stream projects to do the same.

### ⛏ Have you considered any alternatives or workarounds?

We can leave it as is, but that will likely cause problems.

### 📚 Any additional context?

_No response_

Start by searching the repository for every mention of ndjson and its existing file extensions; no specific files or tests are named in the issue. Update the in-repository references to jsonl and verify the old terminology and extensions are gone, while tracking downstream projects separately.

Written by the indexing model from the issue text.