Parallelize 09_issues seeder

What does this MR do and why?

Parallelize 09_issues seeder

This parallelizes the 09_issues development fixture seeder, which speeds it up on macOS (M3 Max) from ~50s to ~21s.

References

• Speed up GDK setup: optimize development fixtur... (&19584 - closed)

How to verify locally

1. (yes | gdk reset-data) > reset-data.txt 2>&1
   • ⚠️ This resets your local databases
2. Verify everything was seeded, but faster

== Seed from /home/peter/devel/gitlab/gdk/gitlab/db/fixtures/development/09_issues.rb
[... many dots and warnings ...]

twitter/Typeahead.Js: 10 issues created!

OK
== /home/peter/devel/gitlab/gdk/gitlab/db/fixtures/development/09_issues.rb took 20.37 seconds

3. Load schema for your test DB: RAILS_ENV=test be rake db:schema:load
   • Due to `gdk reset-data` does not restore test database (gitlab-development-kit#2984)

MR acceptance checklist

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