Adopt label renaming from ~"feature proposal" to ~feature

What does this MR do?

Adopt label renaming from ~"feature proposal" to feature

What are the relevant issue?

gitlab-org/quality/triage-ops#96 (closed)

What are the relevant merge requests?

• gitlab-org/quality/triage-ops!84 (merged)
• gitlab-org/gitlab-insights!63 (merged)
• !18117 (merged)
• https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/24371