Dashboard: don't use subquery (#12387)

Django 4.2 added support for slices when prefetching. This results in a
better query plan ` Sort (cost=287.47..287.52 rows=18 width=1420)` -> `
Subquery Scan on qualify (cost=73.48..74.02 rows=12 width=1420)`, and
also performs better in production

```
# before
6.07 s ± 10.7 ms per loop (mean ± std. dev. of 5 runs, 5 loops each)
# after
4.61 s ± 197 ms per loop (mean ± std. dev. of 5 runs, 5 loops each)
```

Author: stsewd

readthedocs/projects/querysets.py

@@ -3,10 +3,8 @@
 from django.conf import settings
 from django.db import models
 from django.db.models import Count
-from django.db.models import OuterRef
 from django.db.models import Prefetch
 from django.db.models import Q
-from django.db.models import Subquery
 
 from readthedocs.core.permissions import AdminPermission
 from readthedocs.core.querysets import NoReprQuerySet
@@ -143,12 +141,9 @@ def prefetch_latest_build(self):
         from readthedocs.builds.models import Build
 
         # Prefetch the latest build for each project.
-        subquery = Subquery(
-            Build.internal.filter(project=OuterRef("project_id")).order_by("-date").values("pk")[:1]
-        )
         latest_build = Prefetch(
             "builds",
-            Build.objects.filter(pk__in=subquery).select_related("version"),
+            Build.internal.select_related("version").order_by("-date")[:1],
             to_attr=self.model.LATEST_BUILD_CACHE,
         )
         return self.prefetch_related(latest_build)