Unify 50% suffix for utility classes.

Initial conversation for context: gitlab-org/gitlab-ui!2608 (comment 809964310)

Utility classes use -half or -50p for 50% value. We should pick one and unify them for consistency.

Which one is your favourite and why?

• 🌗 -half
• 5️⃣ -50p
• 🌬️ -1/2