Migrate datasource gitlab_projects from SDK to Framework plugin

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On attempting to migrate gitlab_projects to the framework plugin, I discovered it could only be achieved with a breaking change.

Terraform attribute names must start with a letter. This datasource has an attribute called _links, which needs to be renamed as part of the migration.

I pushed up what I had done so far until I discovered the breaking change into a closed draft MR: !2742 (closed)

This is a massive datasource, so even though this will be somewhat out of date by 19.0, it should hopefully help as a starting point.