DEPR: 'epoch' date format in to_json
- [x] closes #57063 
- [x] [Tests added and passed](https://pandas.pydata.org/pandas-docs/dev/development/contributing_codebase.html#writing-tests) if fixing a bug or adding a new feature
- [ ] All [code checks passed](https://pandas.pydata.org/pandas-docs/dev/development/contributing_codebase.html#pre-commit).
- [ ] Added [type annotations](https://pandas.pydata.org/pandas-docs/dev/development/contributing_codebase.html#type-hints) to new arguments/methods/functions.
- [x] Added an entry in the latest `doc/source/whatsnew/vX.X.X.rst` file if fixing a bug or adding a new feature.

Thanks for taking a look at this! @WillAyd now that I am warning for the usage of default `date_format`, some examples in the user guide are failing such as:
```python
dfj2 = dfj.copy()
dfj2["date"] = pd.Timestamp("20130101")
...
dfj2.to_json("test.json")
  ```
  What am I supposed to do in this case? Is there a way to suppress this warning without passing `date_format="iso"`?  Thanks @Aloqeely 