Standardise title_area slots to kebab-case

The following discussion from !42963 (merged) should be addressed:

• @rob.hunt started a discussion: (+1 comment)

  thought (non-blocking): All the other slots use a dash or camelcase based naming convention. Maybe we should update the shared titleArea component to follow a similar convention (separate issue)? 🤔 Even better, maybe we should pick one convention and stick with it across the codebase 😅