Rename UPM target to a more generic name

[vaind]

Since the UPM target is not really Unity-specific (and is going to be used for Unreal package publishing as well), we could find a more fitting name? The script basically gets a zip file and overwrites a target github repository with the contents of that file. So the actual target is "github-repo" - would that be a good name?

FYI @bruno-garcia

[bruno-garcia]

No strong opinions here, maybe @bitsandfoxes has some

[bitsandfoxes]

I just want to point out the level of fitting: The U in UPM could be Unity or Unreal. Still works!
But yes, github-repo works for me.