Quarantine two flaky specs

What does this MR do and why?

This MR quarantines the following flaky specs that were reported in recent broken master incidents, until the flakiness issue is resolved:

• spec/features/invites_spec.rb
• spec/frontend/ci/runner/admin_runners/admin_runners_app_spec.js

Related to #414971 (closed) and #414975 (closed)

MR acceptance checklist

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