[5.x] Changing Stache index name from collection to collectionHandle

[Krzemo]

Fixes #11323

A chain of though here ....

1. Collection filtering in EntryQueryBuilder.php:

public function where($column, $operator = null, $value = null, $boolean = 'and')
{
    if ($column === 'collection') {
        $this->collections[] = $operator;
        return $this;
    }
    // ...
}

When someone does a collection query {{ collection:blog }} (Entry::query()->where('collection', 'blog')), the collection condition is intercepted and stored in a separate $collections property instead of being handled as a normal where clause.

2. Then in the status queries in EntryQueryBuilder.php:

protected function addCollectionWhereToStatusQuery($query, $collection): void
{
    // Using collectionHandle instead of collection because we intercept collection
    // and put it on a property. In this case we actually want the indexed value.
    $query->where('collectionHandle', $collection);
}

The code explicitly uses 'collectionHandle' because it needs to bypass the interception of 'collection' queries that happens in the where() method.

Seems like we should only have 'collectionHandle' because:

1. It's what's actually used for index-based lookups
2. 'collection' queries get handled through a different mechanism
3. Having both would duplicate data unnecessarily
4. Creating it during Stache::warm() ensures it's available when needed for status queries

I hope Im right here ...