Run releases feature specs with feature flag enabled and disabled

Nathan Friendrequested to merge
nfriend-update-releases-index-rspec-for-apollo-client-ff into master

What does this MR do?

Updates the feature specs in spec/features/projects/releases/user_views_releases_spec.rb which cover the main Releases page to run twice: once with the releases_index_apollo_client feature flag enabled, and once with the flag disabled.

Once the releases_index_apollo_client feature flag and old code is removed, this change will be undone, and the tests will go back to only being run once.

Feature flag rollout issue: #331006 (closed)

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