Update BulkImports transformers to remove usage of then

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Suggestion from !68873 (comment 671872442)

Do not use then in BulkImports transformers as it returns new copy of a hash, which contributes to memory consumption increase. Mutate local hash instead.

Example: https://gitlab.com/gitlab-org/gitlab/blob/d8db3d37859d8d7795dfc85e8e3853821c55d628/lib/bulk_imports/groups/transformers/group_attributes_transformer.rb#L10-L10

Update all of the transformers where it's used.