Migrate font weight utility to Tailwind in *.rb

What does this MR do and why?

Related #456641 (closed)

This migrates font-weight utilities to Tailwind variant in *.rb files.

Migrate

• gl-font-weight-semibold -> gl-font-semibold
• gl-font-weight-bold -> gl-font-bold

MR acceptance checklist

Please evaluate this MR against the MR acceptance checklist. It helps you analyze changes to reduce risks in quality, performance, reliability, security, and maintainability.

Screenshots or screen recordings

gl-font-semibold in app/helpers/events_helper.rb

gl-font-bold in app/helpers/merge_requests_helper.rb

How to set up and validate locally