Add revoke achievement mutation and service

What does this MR do and why?

Related to #385393 (closed)

Add the ability to revoke an achievement (via GraphQL).

Screenshots or screen recordings

How to set up and validate locally

Enable feature flag in rails console: Feature.enable(:achievements)

Find some user achievements (you may need to run the seed depending on the age of your database: bundle exec rake db:seed_fu FILTER=36_achievements)

query userAchievements(
  $id: UserID!
) {
  user(id: $id) {
    userAchievements {
      nodes {
        id
        createdAt
        achievement {
          id
          name
          avatarUrl
          namespace {
            id
            name
          }
        }
      }
    }
  }
}
variables
{
  "id": "gid://gitlab/User/1"
}

Try and revoke one:

mutation {
  achievementsRevoke(input: {
    userAchievementId: "gid://gitlab/Achievements::UserAchievement/94" }) {
    userAchievement {
      id
      achievement {
        id
        name
      }
      revokedByUser {
        username
      }
      revokedAt
    }
    errors
  }
}

Try revoking again and check. you get an error: This achievement has already been revoked

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.