Manage achievement order on profile

What does this MR do and why?

Manage achievement order on profile

Changelog: added

Screenshots or screen recordings

No UI changes

How to set up and validate locally

1. Feature.enable(:achievements)
2. Log in and have achievements bin/rake db:seed_fu FILTER=36_achievements
3. Get GIDs for Achievements::UserAchievement

   query {
     user(username: "root") {
       userAchievements {
         nodes {
           id
           achievement { name }
           priority
         }
       }
     }
   }

4. Update the priority for some achievements

   mutation {
     userAchievementPrioritiesUpdate(input: {
       userAchievementIds: ["gid://gitlab/Achievements::UserAchievement/<first user achievement id>", "gid://gitlab/Achievements::UserAchievement/<second user achievement id>"],
     }) {
       userAchievements {
         id
         priority
       }
       errors
     }
   }

5. Execute the query from point 3 again and verify that the achievements are sorted by their priority property (smallest value first, nulls last)

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 #390389