Hide export button for pending promotion tab

What does this MR do and why?

It hides the export csv button when the pending promotion tab is active.

Note: the change is behind a feature flag.

MR acceptance checklist

Please evaluate this MR against the MR acceptance checklist. It helps you analyze changes to reduce risks in quality, performance, reliability, security, and maintainability.

Screenshots or screen recordings

Before	After

How to set up and validate locally

• Enable member_promotion_management feature flag
• Enable ApplicationSetting.first.update!(enable_member_promotion_management: true) application setting
• Use an Ultimate license, and simulate Ultimate SM setup (e.g. export GITLAB_SIMLATE_SAAS=0 gdk restart)
• Visit any Group as an owner of that group (e.g. http://127.0.0.1:3000/groups/gitlab-org/-/group_members)
• Promoting a non-billable GUEST member to a DEVELOPER role (this member should be non-billable on the whole instance, i.e. have a highest role as guest)
• You should see a banner saying "Role change request was sent to the administrator." — this means a promotion request has been created
• Navigate to the group's members
• Visit the "Promotions" tab

Related to #433174 (closed)