Refactor out Dir.glob from ActionDispatch::Static

Dir.glob can be a security concern. The original use was to provide logic of fallback files. Example a request to `/` should render the file from `/public/index.html`. We can replace the dir glob with the specific logic it represents. First by determining if the path represents a directory, in this case appending `index` file name to the path and second if the path does not contain an extension we can add a default extension.

Related to #16464