Use URI.open instead Kernel#open

With Ruby 2.7, Kernel#open was deprecated and URI.open should be used.

Example:

$ bundle exec rspec -c -f d spec -t "${RSPEC_TAGS}"
[...]
/builds/gitlab/spec/gitlab_test_helper.rb:206: warning: calling URI.open via Kernel#open is deprecated, call URI.open directly or use URI#open
[...]
/builds/gitlab/spec/features/backups_spec.rb:72: warning: calling URI.open via Kernel#open is deprecated, call URI.open directly or use URI#open
[...]
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6 examples, 0 failures

https://gitlab.com/gitlab-org/charts/gitlab/-/jobs/1129536148