Deprecate datasource gitlab_projects._links

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This change adds a new links field to replace the existing _links field in the GitLab projects data source. The old _links field is now marked as deprecated and will be removed in version 19.0, while the new links field provides the same functionality with a cleaner name. Both fields currently work the same way and contain project-related links, but users should start using links instead of _links to prepare for the future removal. The documentation has also been updated to reflect this new field.

This is required before the datasource can be migrated to the Framework plugin. Attribute names can only start with a letter, so _links is invalid.