[6.x] Use ResolveValues to resolve values in EloquentQueryBuilder::pluck()

[duncanmcclean]

This PR attempts to fix an issue with the Eloquent Driver where an error would occur when updating entries in a search index.

We're plucking the reference from entries in the entries searchable here:

cms/src/Search/Searchables/Entries.php

Line 46 in 8563d35

return $query->pluck('reference');

However, when entries are stored in the database, this results in an array of null values because reference isn't a "real" column in the database.

This PR adopts the fix suggested in statamic/eloquent-driver#531, where we use ResolveValues to resolve values in EloquentQueryBuilder::pluck(), allowing it to actually hit `Entry::reference().

Fixes statamic/eloquent-driver#531