DEPR: Deprecate cols in several functions

closes #6680 -- Deprecate `cols` in `drop_duplicates()` and `duplicated()` (rename to `subset`)
closes #6645 -- Deprecate `cols` in `to_csv()` and `to_excel()` (rename to `columns`)
releated to #6686 
Related #5505 -- Deprecate `cols` and `rows` in `pivot_table()` and `crosstab()` (rename to `columns` and `index`)

Related #6581 -- Reminder to come back in 0.16, remove deprecation warnings, and use new arguments