Renamed and added lemmas for setD

[Brixfoly]

Motivation for this change

The lemma bigcupDr was not that well named compared to setD_bigcupl for example.
Plus the equality was not in the same way, and as a result everytime it was rewrited in mathcomp,
it was with a -. This PR renames it, adds a similar lemma for bigcap (because I will need it in my work)
and renames everywhere the previous name was used.

Checklist

• added corresponding entries in CHANGELOG_UNRELEASED.md

• added corresponding documentation in the headers

Reference: How to document

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