Revert "fix: allow update milestone start/end dates"

This reverts commit 64a6e10.

Author: maxceem

src/routes/milestones/update.js

@@ -22,9 +22,9 @@ const schema = {
     name: Joi.string().max(255).optional(),
     description: Joi.string().max(255),
     duration: Joi.number().integer().min(1).optional(),
-    startDate: Joi.date().allow(null),
+    startDate: Joi.any().forbidden(),
     actualStartDate: Joi.date().allow(null),
-    endDate: Joi.date().min(Joi.ref('startDate')).allow(null),
+    endDate: Joi.any().forbidden(),
     completionDate: Joi.date().allow(null),
     status: Joi.string().max(45).optional(),
     type: Joi.string().max(45).optional(),