Allow to delete UserAchievements

What does this MR do and why?

Allow to delete UserAchievements

Screenshots or screen recordings

No UI involved

How to set up and validate locally

1. In rails console enable the feature

   Feature.enable(:achievements)

2. Find a user achievement Achievements::UserAchievement.first and get its ID
3. Execute the mutation with an admin user or a user with owner role in the namespace where it is created

   mutation {
     userAchievementsDelete(input: {
       userAchievementId: "gid://gitlab/Achievements::UserAchievement/<ID>"
     }) {
       errors
       userAchievement {
         id
       }
     }
   }

4. Verify that the UserAchievement does no longer exist

MR acceptance checklist

This checklist encourages us to confirm any changes have been analyzed to reduce risks in quality, performance, reliability, security, and maintainability.

• I have evaluated the MR acceptance checklist for this MR.

Related to #414990 (closed)