Fix Rake/Require offense in spdx rake task

What does this MR do and why?

This MR moves all requires into task definition of spdx rake task.

Contributes to #391459 (closed).

See !101740 (merged)

How to set up and validate locally

rake gitlab:spdx:import

Still works ✅

$ bundle exec rubocop ee/lib/tasks/gitlab/spdx.rake
Inspecting 1 file
.

1 file inspected, no offenses detected

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