Update gitlab-dangerfiles to 3.0.0 which import plugins/rules automatically

What does this MR do and why?

This update to gitlab-dangerfiles 3.0.0 which includes gitlab-org/ruby/gems/gitlab-dangerfiles!118 (merged), removing the need to manually import custom Dangerfiles and plugins.

This also brings the danger_local Rake task from the gem.

How to set up and validate locally

Run bundle exec rake danger_local.

MR acceptance checklist

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