depr: enforce ignore_nulls default change in 1.0

[MarcoGorelli]

I'm just opening an issue about this so it can be added to the 1.0 milestone without it being forgotten

#14663

the default for ignore_nulls will change from True to False

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reasons for enforcing this in 1.0 (as opposed to 2.0):

• as shown in https://github.com/pola-rs/polars/pull/14663/files, the docs originally documented False to be the default, even though it was True, which suggests an implementation oversight
• this function matches pandas quite closely, and the default there is ignore_na=False

[stinodego]

This will be closed as part of #13525

But keeping it around as an issue just to be sure is fine I guess.