Speed up rubocop CI jobs

Why

To speed up rubocop CI job.

What

This MR:

• separates rubocop and its related gems into new group named :rubocop
• removes loading rubocop from rake tasks (will make other jobs with bundle exec rake than rubocop faster a bit)
• overrides bundle install in before_script only for rubocop job by --without default test
• enables parallel execution of rubocop
• (adds credit for previous MR !2297 (merged))

Test

Items	Before	After	Changes
rubocop job duration	308 sec	50 sec	6x times faster
rubocop target files	Inspecting 462 files	Inspecting 462 files	The number of examined files does not change.

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Closes #3268 (closed)