Update app/models/user.rb to stop removing spam_logs associated with a user during destroy

What does this MR do?

We currently remove Spam Logs upon destroying a user as per https://gitlab.com/gitlab-org/gitlab/-/blob/master/app/services/users/destroy_service.rb#L41 and https://gitlab.com/gitlab-org/gitlab-foss/-/blob/master/app/models/user.rb#L177

      # Calling all before/after_destroy hooks for the user because
      # there is no dependent: destroy in the relationship. And the removal
      # is done by a foreign_key. Otherwise they won't be called
      user.members.find_each { |member| member.run_callbacks(:destroy) }

  has_many :spam_logs,                dependent: :destroy # rubocop:disable Cop/ActiveRecordDependent

This MR is intended to discuss a change to this behavior as I think we should never remove spam_logs. Those are logs after all and in that sense they don't necessarily reflect the current state of the system but rather past events, regardless of whether or not users responsible for that exist or not

https://gitlab.slack.com/archives/CSXDTD2TE/p1623929590067500

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