Quarantine flaky specs

Ref: #370812 (closed)

What does this MR do and why?

As suggested in Flaky tests, this MR quarantines flaky specs in ee/spec/services/users/abuse/git_abuse/namespace_throttle_service_spec.rb and ee/spec/services/users/abuse/excessive_projects_download_ban_service_spec.rb until we figure out how to fix them.

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