[prdoc] Require SemVer bump level (paritytech#3816)

A prerequisite for adding a stable branch and respecting SemVer on new
stable releases is including SemVer bump levels in our PRDocs.

Next release is scheduled for April 3rd, so it would be great to get
this merged before then.

Also added "None" as a valid bump option, to support test/benchmark
changes and CI to ensure changed crates have an entry.

prdoc/schema_user.json

@@ -132,7 +132,8 @@
           }
         },
         "required": [
-          "name"
+          "name",
+          "bump"
         ]
       },
       "migration_db": {
@@ -187,6 +188,11 @@
             "const": "patch",
             "title": "Patch",
             "description": "A bump to the third leftmost non-zero digit of the version number."
+          },
+          {
+            "const": "none",
+            "title": "None",
+            "description": "This change requires no SemVer bump (e.g. change was a test or benchmark)."
           }
         ]
       },