spec: Make `--footnote=dpub` the default

[spring-raining]

Goals

In #231, an option has been added to specify the behavior when converting footnotes to HTML. I propose making footnote=dpub the default setting in the next major version update.

This discussion is transferred from vivliostyle/vivliostyle-cli#758

[akabekobeko]

memo:
開発者会議 April 2026 にて議論。デフォルト値の変更 = 破壊的変更 (というほどではないが) なので、remark 更新と一緒に major version up 対応でよさそう。