Add new invite_email column index on members for new query

What does this MR do and why?

Add new invite_email column index on members for new query

• needed for work in !152275 (merged)
  • separate MR than the code as per docs - https://docs.gitlab.com/ee/development/database/adding_database_indexes.html#new-or-updated-queries-perform-slowly-on-gitlabcom
  • see that MR for the query used and the query plans
• see #457938 (closed)

Changelog: other

DB query analysis

• old query: https://console.postgres.ai/gitlab/gitlab-production-tunnel-pg12/sessions/28128/commands/87605
• new query with index: https://console.postgres.ai/gitlab/gitlab-production-tunnel-pg12/sessions/28128/commands/87595
• further analysis: !152275 (comment 1896748845) and !152275 (comment 1896780730)

MR acceptance checklist

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Related to #457938 (closed)