fix(db-mongodb): joins with singular collection name (#8933)

### What?
Properly specifies `$lookup.from` when the collection name is singular.

### Why?
MongoDB can pluralize the collection name and so can be different for
singular ones.

### How?
Uses the collection name from the driver directly
`adapter.collections[slug].collection.name` instead of just `slug`.

test/joins/collections/Categories.ts

@@ -1,6 +1,7 @@
 import type { CollectionConfig } from 'payload'
 
 import { categoriesSlug, postsSlug } from '../shared.js'
+import { singularSlug } from './Singular.js'
 
 export const Categories: CollectionConfig = {
   slug: categoriesSlug,
@@ -83,5 +84,11 @@ export const Categories: CollectionConfig = {
         },
       ],
     },
+    {
+      name: 'singulars',
+      type: 'join',
+      collection: singularSlug,
+      on: 'category',
+    },
   ],
 }