pd.to_datetime() fails for KeyError when raising monotonic increasing index ValueError
- [x] closes #28238
- [x] tests added / passed
- [x] Ensure all linting tests pass, see [here](https://pandas.pydata.org/pandas-docs/dev/development/contributing.html#code-standards) for how to run them
- [ ] whatsnew entry

Test added to test_to_datetime.py to test the conversion to datetime for a series with a float index.
thanks @usersblock 