DOC: Fix capitalization among headings in doc/source/whatsnew (#32550)
Updated headers for the following files:

```
- [x] doc/source/whatsnew/v0.19.0.rst
- [x] doc/source/whatsnew/v0.20.0.rst
- [x] doc/source/whatsnew/v0.24.0.rst
- [x] doc/source/whatsnew/v0.25.0.rst
- [x] doc/source/whatsnew/v1.0.0.rst
```

thanks @themien 