Optimize AnalyticsPostCount (#20727)

We use * instead of a column name to use
index-only scan always even when other column filters
are applied.

Right now, index-only scan will only get applied
in the basic query of "select count(p.id) as value
from posts p". But it won't get applied if the query
is "select count(p.id) as value from posts p where
p.deleteat=0".

So this is a minor optimization which improves
some corner cases.

This was found from the slow query monitoring.

```release-note
NONE
```

store/sqlstore/post_store.go

@@ -2195,7 +2195,7 @@ func (s *SqlPostStore) AnalyticsPostCountsByDay(options *model.AnalyticsPostCoun
 
 func (s *SqlPostStore) AnalyticsPostCount(options *model.PostCountOptions) (int64, error) {
 	query := s.getQueryBuilder().
-		Select("COUNT(p.Id) AS Value").
+		Select("COUNT(*) AS Value").
 		From("Posts p")
 
 	if options.TeamId != "" {