Fix transient failure of group_member_contributions/components/app_spec.js

What does this MR do?

Move asynchronous fetchContributedMembers() out of synchronous mounted() life cycle hook of group_member_contributions/components/app.vue. That way the corresponding component test does not transiently fail when the request is still pending.

See 7 failed request to /foo/analytics.json before this change (https://gitlab.com/gitlab-org/gitlab-ee/-/jobs/117098883) and none afterwards (https://gitlab.com/gitlab-org/gitlab-ee/-/jobs/117780059).

What are the relevant issue numbers?

closes https://gitlab.com/gitlab-org/gitlab-ee/issues/7728 again

Does this MR meet the acceptance criteria?

• Changelog entry added, if necessary
• Documentation created/updated
• Tests added for this feature/bug
• Conforms to the code review guidelines
• Conforms to the merge request performance guidelines
• Conforms to the style guides
• Conforms to the database guides
• Link to e2e tests MR added if this MR has Requires e2e tests label. See the Test Planning Process.
• EE specific content should be in the top level /ee folder
• For a paid feature, have we considered GitLab.com plans, how it works for groups, and is there a design for promoting it to users who aren't on the correct plan?