Rename creator_id to user_id for `remote_development_namespace_cluster_agent_mappings` table
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## Description
As discussed [here](https://gitlab.com/gitlab-org/gitlab/-/merge_requests/145387#note_1842487256), rename column `creator_id` to `user_id` for `remote_development_namespace_cluster_agent_mappings` table to better follow association logic.
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