Add details to guest overage modal

What does this MR do and why?

Adds a confirmation modal before users upgrade Guest users to a higher role. Right now this modal appears every time a guest user's access level is increased. This logic will be refined further in #199893 (closed).

This MR is iteration 2 of #363498 (closed) which connects the dummy values added in MR !105613 (merged) to the values fetched via graphQL query.

This change is currently behind the feature flag show_overage_on_role_promotion.

Screenshots or screen recordings

Scenario	Screenshot	Scenario	Screenshot
Group members page, promoting member		Group members page, promoting group
Project members page, promoting member		Project members page, promoting group

How to set up and validate locally

• Open up the rails console rails c and enable the show_overage_on_role_promotion flag (Feature.enable(:show_overage_on_role_promotion))
• Navigate to the project members page (<group-name>/<project-name>/-/project_members) or group members page (groups/<group-name>/-/group_members)
• Change the Max role value for a member (in members tab) or a group (in groups tab), the modal should appear
  • Clicking off of the modal or clicking "Cancel" should close the modal and NOT update the role of the user or group.
  • Clicking "Continue with overages" will close the modal and update the role of the user or group.
• Clicking the "Learn more" link should open a help page in a new tab.
• Open the Max role dropdown and select Guest or any other role. The modal should not be shown in this case.

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 #363498 (closed)