Disable pull mirror if repository is in read-only state

Stan Hurequested to merge
sh-guard-repository-mirrors-read-only into master

During a project migration to another shard in https://gitlab.com/gitlab-com/gl-infra/production/issues/1250, we noticed the GetAllLFSPointers RPC was timing out for a repository that was marked as read-only.

To avoid updates while the migration is happening, we need to check the repository_read_only flag and skip the update.

Edited by Stan Hu

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