Replaced silver occurrences with premium

doc/development/licensed_feature_availability.md

ee/lib/gitlab/expiring_subscription_message.rb

ee/spec/features/admin/groups/admin_changes_plan_spec.rb

ee/spec/features/billings/billing_plans_spec.rb

ee/spec/finders/ee/projects_finder_spec.rb

ee/spec/finders/groups_with_templates_finder_spec.rb

ee/spec/fixtures/gitlab_com_plans.json

ee/spec/frontend/subscriptions/new/store/state_spec.js

ee/spec/helpers/billing_plans_helper_spec.rb

ee/spec/lib/ee/gitlab/application_context_spec.rb

ee/spec/lib/gitlab/expiring_subscription_message_spec.rb

ee/spec/models/ee/namespace_spec.rb

ee/spec/models/ee/user_spec.rb

ee/spec/models/elasticsearch_indexed_namespace_spec.rb

ee/spec/models/gitlab_subscription_spec.rb

ee/spec/models/project_spec.rb

ee/spec/models/push_rule_spec.rb

ee/spec/policies/group_policy_spec.rb

ee/spec/requests/api/namespaces_spec.rb

ee/spec/services/dashboard/projects/list_service_spec.rb