Rename destination_name column to destination_slug for bulk imports on db level

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Related to #365166 (closed)

We added a new field to Bulk Imports API destination_slug to be used in favour of destination_name. However, on the database level, the column is still called destination_name. We should rename it on the database level for consistency.

In the related issue, @carlad-gl suggested an implementation plan:

• add a destination_slug column to the bulk_import_entities table on the database
• populate each row's destination_slug with the data from the destination_name column
• change any occurrence of destination_slug in the bulk_importer and api to destination_path, for both Groups and Projects
• delete the destination_name column
• update the api documentation (both REST and GraphQL)
• update the UI (see #347510) to clarify that the input field is for a slug not a name