Migrate resource gitlab_project_milestone from terraform SDK to Framework plugin

Prompt: Migrate the gitlab_project_milestone resource (defined in internal/provider/sdk/resource_gitlab_project_milestone.go) from the old SDK to the new terraform Framework plugin.

Acceptance Criteria: Functionality for the Read/Create/Update/Delete functions should be the same as the old SDK resource. Tests should be equal between the two resources. There should be a new test checking that users can migrate from the old to the new version without errors.