Remove deprecated attributes 2

This merge request removes deprecated fields and attributes from the GitLab Terraform provider as part of preparing for version 19.0. The changes include:

Renaming for consistency: The public_builds field is renamed to public_jobs to better reflect what it actually controls - whether CI/CD jobs can be viewed by non-project members.

Removing deprecated boolean fields: Several old true/false settings like issues_enabled, merge_requests_enabled, pipelines_enabled, wiki_enabled, and snippets_enabled are removed. These have been replaced by more flexible access level controls that offer options like "disabled", "private", or "enabled" instead of just on/off.

Simplifying project membership: The data source for project membership now only uses a single project field instead of having multiple deprecated ways to specify the same thing (project_id and full_path).

Updating protected environments: The way deploy access levels are configured for protected environments is simplified to use a single consistent attribute name.

Documentation updates: All the documentation is updated to reflect these changes and includes a migration guide showing users exactly how to update their configurations for the new version.

These changes make the provider's interface cleaner and more consistent while removing old functionality that was confusing or redundant.

Part of #6512