Use project namespace to retrieve project members

What does this MR do and why?

On the project members page (e.g. https://gitlab.com/gitlab-org/gitlab/-/project_members), retrieve the project members through the project namespace relationship instead of the polymorphic source relationship. There is a good explanation of the overall context in the issue description.

SQL Query Plans

15846663 is the ID of the project namespace for the gitlab project.

9970 is the gitlab-org group

project.namespace_members

SELECT 
  "members".* 
FROM 
  "members" 
WHERE 
  "members"."type" = 'ProjectMember' 
  AND "members"."member_namespace_id" = 15846663 
  AND "members"."requested_at" IS NULL

Query plan: https://postgres.ai/console/gitlab/gitlab-production-tunnel-pg12/sessions/14090/commands/49219

---

project.namespace_requesters

SELECT 
  "members".* 
FROM 
  "members" 
WHERE 
  "members"."type" = 'ProjectMember' 
  AND "members"."member_namespace_id" = 15846663 
  AND "members"."requested_at" IS NOT NULL

Query plan: https://postgres.ai/console/gitlab/gitlab-production-tunnel-pg12/sessions/14090/commands/49220

---

group.namespace_members

SELECT 
  "members".* 
FROM 
  "members" 
WHERE 
  "members"."type" = 'GroupMember' 
  AND "members"."member_namespace_id" = 9970 
  AND "members"."requested_at" IS NULL 
  AND "members"."access_level" != 5

Query plan: https://postgres.ai/console/gitlab/gitlab-production-tunnel-pg12/sessions/14090/commands/49221

---

group.namespace_requesters

SELECT 
  "members".* 
FROM 
  "members" 
WHERE 
  "members"."type" = 'GroupMember' 
  AND "members"."member_namespace_id" = 9970 
  AND "members"."requested_at" IS NOT NULL

Query plan: https://postgres.ai/console/gitlab/gitlab-production-tunnel-pg12/sessions/14090/commands/49222

MR acceptance checklist

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