Applied configuration is not stored properly due to incorrect array operator

Problem/Motivation

I created a config-modify file in a custom module and it should be executed after they installation. When the module is enabled the configuration is being applied, but the filename isn't be registered as applied and when executed again, the configuration is duplicated.

The error happen at: html/modules/contrib/config_modify/src/ConfigInstaller.php:142

$alterations_applied->set("files", $alterations_applied->get("files") + $config_to_alter)->save();

Looking this documentation: https://www.amitmerchant.com/two-ways-to-merge-arrays-in-php/, should be better use array_merge for this code, because using "+" the union is assossiative and can hide some items when both array has the same key.

Steps to reproduce

• Create config-modify file in a module
• Execute config-modify and check the applied-configuration
• As the configuration isn't be saved at applied-configuration, it will be executed again and duplicated the configuration

Proposed resolution

Replace "+" as union and use array_merge function.

Remaining tasks

N/A

User interface changes

N/A

API changes

N/A

Data model changes

N/A