Following Google's move of the source, the new GitHub version can be accessed via jsDelivr (a content deliver network with no bandwidth limits that's focused on performance, reliability, and security) by using the following script urls (standard and packed versions):
https://cdn.jsdelivr.net/gh/googlemaps/v3-utility-library@markermanager/1.2/markermanager.js
https://cdn.jsdelivr.net/gh/googlemaps/v3-utility-library@markermanager/1.2/markermanager_packed.js
These urls specifically target the 1.2 release of the markermanager library - as covered in the following SO answer, in production you should consider targeting a specific release tag to ensure you're getting a specific release version of the script:
Link and execute external JavaScript file hosted on GitHub
Alternatively, you could download and include the library directly in your project for production purposes.