DOC: Prepare release notes for 2.1.0rc0 and remove 2.0.4 release notes
- [ ] closes #xxxx (Replace xxxx with the GitHub issue number)
- [ ] [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.
- [ ] Added an entry in the latest `doc/source/whatsnew/vX.X.X.rst` file if fixing a bug or adding a new feature.

```
2023-08-10T21:21:21.8021565Z /home/runner/work/pandas/pandas/doc/source/whatsnew/index.rst:38: WARNING: toctree contains reference to nonexisting document 'whatsnew/v2.0.4'
```