Migrate "placement" values to match placements use in floating-ui

The following discussion from !4061 (merged) should be addressed:

• @mrincon started a discussion: (+2 comments)

  Note: I find this remapping/renaming unfortunate as it would be hard for us to add true "right" placement as in floating UI.

  This is not a blocker to add right-start as done here, but it does make the mapping confusing. 🤔