Migrate resource gitlab_integration_external_wiki from terraform SDK to Framework plugin

Prompt: Migrate the gitlab_integration_external_wiki resource (defined in internal/provider/sdk/resource_gitlab_integration_external_wiki.go) from the old SDK to the new terraform Framework plugin. Delete the old files.

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. The old version should be deleted.