fix(routes): remove validation for milestone parameter as it cannot be derived from octokit/routes

BREAKING CHANGE: the `milestone` parameter is no longer validated using a regex as it cannot be derived from [octokit/routes](https://github.com/octokit/routes)

Author: gr2m

plugins/rest-api-endpoints/routes.json

@@ -3791,8 +3791,7 @@
           "type": "string"
         },
         "milestone": {
-          "type": "string",
-          "validation": "^([0-9]+|none|\\*)$"
+          "type": "string"
         },
         "owner": {
           "required": true,