Add full environment to rake gitlab:elastic:index

What does this MR do and why?

This MR fixes a ~"technical debt" issue. Currently, we don't have a full rails environment for rake gitlab:elastic:index. Now it works by accident because we invoke other rake tasks with the :environment flag before invoking the rails code. For example, if we remove these lines, the rake task will fail with

$ bin/rake gitlab:elastic:index
rake aborted!
NameError: uninitialized constant Gitlab::SafeRequestStore
/Users/dgruzd/projects/gdk-ee/gitlab/lib/gitlab/current_settings.rb:15:in `current_application_settings'
/Users/dgruzd/projects/gdk-ee/gitlab/lib/gitlab/current_settings.rb:32:in `method_missing'
/Users/dgruzd/projects/gdk-ee/gitlab/ee/lib/tasks/gitlab/elastic.rake:16:in `block (3 levels) in <main>'
Tasks: TOP => gitlab:elastic:index
(See full trace by running task with --trace)

which can be very confusing. I think we should explicitly require full environment for this rake task.

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