Update past major versions to new '.0' style and add Swift 6.0 (#557)

Author: dempseyatgithub

assets/javascripts/swift-evolution.js

@@ -18,13 +18,13 @@ var proposals
  */
 var languageVersions = [
   '2.2',
-  '3',
+  '3.0',
   '3.0.1',
   '3.1',
-  '4',
+  '4.0',
   '4.1',
   '4.2',
-  '5',
+  '5.0',
   '5.1',
   '5.2',
   '5.3',