[4.x] Fix dropRLSPolicies() (#1413)

`?` parameters are not supported in these statements, so we have to use
string interpolation like in other related code.

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Co-authored-by: Samuel Štancl <samuel@archte.ch>

src/Concerns/ManagesRLSPolicies.php

@@ -26,7 +26,7 @@ trait ManagesRLSPolicies
         $policies = static::getRLSPolicies($table);
 
         foreach ($policies as $policy) {
-            DB::statement('DROP POLICY ? ON ?', [$policy, $table]);
+            DB::statement("DROP POLICY {$policy} ON {$table}");
         }
 
         return count($policies);