Build: Rename the primary branch master to main

Closes gh-1944

Author: mgol

CONTRIBUTING.md

@@ -53,10 +53,10 @@ cd jquery-ui
 git remote add upstream git://github.com/jquery/jquery-ui.git
 ```
 
-* Get in the habit of pulling in the "upstream" master to stay up to date as jQuery UI receives new commits.
+* Get in the habit of pulling in the "upstream" main branch to stay up to date as jQuery UI receives new commits.
 
 ```bash
-git pull upstream master
+git pull upstream main
 ```
 
 ### Environment: Recommended Setup

package.json

@@ -6,7 +6,7 @@
 	"homepage": "http://jqueryui.com",
 	"author": {
 		"name": "jQuery Foundation and other contributors",
-		"url": "https://github.com/jquery/jquery-ui/blob/master/AUTHORS.txt"
+		"url": "https://github.com/jquery/jquery-ui/blob/main/AUTHORS.txt"
 	},
 	"main": "ui/widget.js",
 	"maintainers": [