Revert "Removed show_trial_status_in_sidebar_experiment_percentage feature flag"

This reverts commit 48b6d01d. Adds the
feature flag definition YAML file back in for the now-merged-in group
trial status sidebar widget experiment:
https://gitlab.com/gitlab-org/gitlab/-/merge_requests/50090

ee/config/feature_flags/experiment/show_trial_status_in_sidebar_experiment_percentage.yml

+---
+name: show_trial_status_in_sidebar_experiment_percentage
+introduced_by_url: https://gitlab.com/gitlab-org/gitlab/-/merge_requests/50090
+rollout_issue_url: https://gitlab.com/gitlab-org/gitlab/-/issues/281019
+milestone: '13.8'
+type: experiment
+group: group::conversion
+default_enabled: false