https://bower.herokuapp.com is the default bower registry used with the version we have for building Mirage2. This registry has been deprected since October 2017 and is now offline / no longer responding to requests. This causes Mirage2 build failures.
As per the announcement / instructions in https://gist.github.com/sheerun/c04d856a7a368bad2896ff0c4958cb00 , we need to update the .bowerrc file in dspace-xmlui-mirage2 for 5.x and 6.x so that the new official registry is used.
We should resolve this before 5.9 and 6.3 are released, and also announce via lists / wiki so that people can resolve without upgrading if needed (it's just a one liner config addition...), so they can still rebuild their existing source