breaking: remove deprecated behavior

[jmoralez]

Removes the following behavior, which has been deprecated for about a year:

• Providing the id as the dataframe index, it has to be a column. Also the outputs from the methods will now always have the id as a column.
• Providing sort_df to fit and cross_validation, we always check if it's sorted.

Also updates the documentation to remove usages of this behavior, docs that use datasetsforecast.losses and uses utilsforecast.plotting.plot_series instead of StatsForecast.plot

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