Update v6 to Floating UI

[mdo]

Replaces #36683 thanks to Claude. Had him port the changes from @GeoSot's original branch and modernize it on top of everything else happening in v6-dev. I haven't reviewed every line here, but a quick poke around the docs locally showed that things were working as intended.

Few things to follow up with:

• [ ] Remove the need to manually enable tooltips and popovers "for performance reasons"
• [ ] Remove navbar static dropdown positioning
• [ ] Consider new components like a more custom select or something else?