Infrastructure: Update script that provides skipto menu on all pages …

…to version 5.1 (#2619)

      Changes included in SkipTo version 5.1:

* Supports including headings and landmarks in custom web components.
* Rewritten to use Javascript  modules.
* Change configuration options to allow customization of container element to be customized; default container is `nav` element.
* The APG implementation of SkipTo sets the container to `div` so it does not create a navigation landmark. 
      
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Co-authored-by: Matt King <[email protected]>