Chapter 6 uses deprecated `append` on Pandas dataframe

[gvwilson]

Line 20-24 of Chapter-6/app.py is currently:

df = (
    df.append({"Year": MIN_YR - 1}, ignore_index=True)
    .sort_values("Year", ignore_index=True)
    .fillna(0)
)

but df.append is not available in Pandas 2.2.2. This code works with that version of Pandas:

df = (
    pd.concat([df, pd.DataFrame([{"Year": MIN_YR - 1}])], ignore_index=True)
    .sort_values("Year", ignore_index=True)
    .fillna(0)
)