Fix project subscriptions policy

What does this MR do and why?

Fix project subscriptions policy

The policy mistakenly required developer access to the downstream project in order to read pipeline subscriptions, causing a problem in the GraphQL endpoint relying on the :read_project_subscription ability.

See #434831 (comment 1736956507)

Changelog: fixed

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• as a Maintainer of a project viewing Settings --> CI/CD --> Pipeline Subscriptions: I can see all the subscribed and supscription projects listed correctly ✅
• as an Owner of a project viewing Settings --> CI/CD --> Pipeline Subscriptions: I can see all the subscribed and supscription projects listed correctly ✅

Further, I tested the graphql query on both the current(master branch and fix-project-subscriptions-policy with:

project(fullPath: "flightjs/Flight") {
    ciSubscribers {
      count
      nodes {
        downstreamProject {
          name
          namespace {
            name
          }
        }
        upstreamProject {
          name
          namespace {
            name
          }
        }
      }
    }
    ciSubscriptions {
      count
      nodes {
        downstreamProject {
          name
          namespace {
            name
          }
        }
        upstreamProject {
          name
          namespace {
            name
          }
        }
      }
    }
  }

##The results

User Type	Before(current behaviour on master)	After (with changes in this MR)
Developer	✅ count --- ❌ subscription or subscribed projects	✅ count --- ❌ subscription or subscribed projects
Maintainer	✅ count --- ❌ subscription or subscribed projects	✅ count --- ✅ subscription + subscribed projects
Owner	✅ count --- ✅ subscription + subscribed projects	✅ count --- ✅ subscription + subscribed projects

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