Upgrade js-yaml to 4.0

The only code change is switching `safeLoad` (which was removed) for `load` (which is "safe" by default now)

https://github.com/nodeca/js-yaml/blob/master/CHANGELOG.md#400---2021-01-03

src/labeler.ts

@@ -101,7 +101,7 @@ async function getLabelGlobs(
   );
 
   // loads (hopefully) a `{[label:string]: string | StringOrMatchConfig[]}`, but is `any`:
-  const configObject: any = yaml.safeLoad(configurationContent);
+  const configObject: any = yaml.load(configurationContent);
 
   // transform `any` => `Map<string,StringOrMatchConfig[]>` or throw if yaml is malformed:
   return getLabelGlobMapFromObject(configObject);