Remove old integration resources 3

This change removes deprecated GitLab integration resources that were scheduled for removal in version 19.0. Specifically, it eliminates the old gitlab_integration_external_wiki, gitlab_integration_jira, and gitlab_integration_mattermost resources along with their documentation and example files. These older resources are being replaced by newer equivalents with gitlab_project_integration_ prefixes that provide the same functionality but with updated naming conventions. The code also simplifies the external wiki integration implementation by removing the backwards compatibility logic that supported both old and new resource names. This is a cleanup effort to streamline the codebase and remove deprecated functionality that users should have already migrated away from.

Part of #6512