Use cached weight for epic board lists

What does this MR do and why?

Uses cached weight counts to get epic lists overall weight. This was the main reason for caching these counts - &7780.

DB queries:

original query (taken from !76440 (merged)):

• before:
  • cold cache: https://postgres.ai/console/gitlab/gitlab-production-tunnel-pg12/sessions/14220/commands/49912
  • warm cache: https://postgres.ai/console/gitlab/gitlab-production-tunnel-pg12/sessions/14220/commands/49913
• after
  • cold cache: https://postgres.ai/console/gitlab/gitlab-production-tunnel-pg12/sessions/14220/commands/49916
  • warm cache: https://postgres.ai/console/gitlab/gitlab-production-tunnel-pg12/sessions/14220/commands/49914

Related to #356686 (partially - cached counts will be used also on other places) and #353450 (closed)

Screenshots or screen recordings

Screenshots are required for UI changes, and strongly recommended for all other merge requests.

How to set up and validate locally

1. Enable the invite modal

   Feature.enable(:epic_board_total_weight)

2. Sample query:

query {
  group(fullPath: "flightjs") {
    id
    epicBoard(id: "gid://gitlab/Boards::EpicBoard/1") {
      lists {
        edges {
          node {
            id
            metadata {
              epicsCount
              totalWeight
            }
          }
        }
      }
    }
  }
}

MR acceptance checklist

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