`DataFrame.pct_change` should default `fill_method=None` instead of `'pad'`
Both `DataFrame.pct_change` and `Series.pct_change` should have default behavior that matches the "obvious" implementation:

```python
lag = series.shift(1)
change = (series - lag) / lag
```

Unfortunately, the default behavior is set to `fill_method='pad'` which surprises many people (#6389). We should start a deprecation cycle to change the default to behave as expected, `fill_method=None`. I don't believe this will be disruptive, as most people think that's how it behaves currently, as far as I know.