Introduce issues_completed_analytics behind feature flag

What does this MR do and why?

As the first of two MRs before adding "Issues Closed" to the Issues Analytics chart for GitLab Ultimate, this MR:

• Creates a new GitLab Ultimate feature called issues_completed_analytics
• Creates a feature flag called issues_completed_analytics_feature_flag
• Removes unsupported tokens from the filter bar when the aforementioned feature flag is enabled and user is a GitLab Ultimate customer due to the issuesCompletedCount query having limited filters at both the group and project levels

Screenshots or screen recordings

Before	After

How to set up and validate locally

1. Make sure you have a GitLab Ultimate license
2. Feature.enable(:issues_completed_analytics_feature_flag)
3. Go to a group/project with issues, and visit http://{LOCAL_INSTANCE}/groups/{GROUP_PATH}/-/issues_analytics or http://{LOCAL_INSTANCE}/{GROUP_PATH}/{PROJECT_PATH}/-/analytics/issues_analytics and verify that the unsupported tokens have been removed from the filter bar, and that it works as expected
4. Feature.disable(:issues_completed_analytics_feature_flag) and repeat the above step, ensuring that the default tokens are present

MR acceptance checklist

This checklist encourages us to confirm any changes have been analyzed to reduce risks in quality, performance, reliability, security, and maintainability.

• I have evaluated the MR acceptance checklist for this MR.

Related to #233905 (closed)