Rails5: fix slice in burndown fixture

What does this MR do?

Fix NoMethodError: undefined method 'slice' for #<ActiveRecord::AssociationRelation []> for rake db:seed_fu for Rails 5.

In Rails 5 blanket array delegation was removed, so open_issues.slice() fails because slice is not in the explicitly delegated methods of Rails. These slice can also be replaced by limit queries.

Can be reproduced locally by running BUNDLE_GEMFILE=Gemfile.rails5 rake db:seed_fu

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Why was this MR needed?

rails5 upgrade

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Does this MR meet the acceptance criteria?

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• Documentation created/updated
• API support added
• Tests added for this feature/bug
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  • Has been reviewed by a Backend maintainer
  • Has been reviewed by a Database specialist
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• End-to-end tests pass (package-and-qa manual pipeline job)

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