Use custom terser gem

What does this MR do?

Use terser from a custom repository that has changes from https://github.com/ahorek/terser-ruby/pull/6. This is temporary until that PR is accepted and a new release is cut.

Closes: omnibus-gitlab#5716 (closed)

Screenshots

Does this MR meet the acceptance criteria?

Conformity

• Changelog entry
• Documentation (if required)
• Code review guidelines
• Merge request performance guidelines
• Style guides
• Database guides
• Separation of EE specific content