Correct doc link for gitlab_project.use_custom_template access token issue

This change updates documentation links in two files to remove version-specific references. The links that previously pointed to GitLab documentation version 15.7 have been simplified to point to the current documentation instead. This ensures the links will always direct users to the most up-to-date information rather than potentially outdated version-specific pages. The actual functionality and warning message about Group Tokens not working with custom templates remains unchanged - only the documentation links were updated.

Related to #6668