Add yml files for two more experiments

ee/config/feature_flags/experiment/invite_members_new_dropdown_experiment_percentage.yml

+---
+name: invite_members_new_dropdown_experiment_percentage
+introduced_by_url: https://gitlab.com/gitlab-org/gitlab/-/merge_requests/50069
+rollout_issue_url: https://gitlab.com/gitlab-org/gitlab/-/issues/268129
+milestone: '13.8'
+type: experiment
+group: group::expansion
+default_enabled: false

ee/config/feature_flags/experiment/trial_onboarding_issues_experiment_percentage.yml

+---
+name: trial_onboarding_issues_experiment_percentage
+introduced_by_url: https://gitlab.com/gitlab-org/gitlab/-/merge_requests/48052
+rollout_issue_url: https://gitlab.com/gitlab-org/gitlab/-/issues/276703
+milestone: '13.8'
+type: experiment
+group: group::conversion
+default_enabled: false