merge error on Roomify, bat_roomify module created

Problem/Motivation

Beginning with Drupal 10.2, the use of Merge::key() with an array of multiple values will cause a deprecation warning.
With Drupal 11, error
"TypeError: Cannot access offset of type array on array in Drupal\Core\Database\Query\Merge->key() (line 331 of core/lib/Drupal/Core/Database/Query/Merge.php).
"
is produced.

Roomify has not been maintained for a while now (last update on Aug 25, 2020)
https://github.com/Roomify/bat/commit/1034b29ee1a54031de87677632888d3873...

Steps to reproduce

Create an event

Remaining tasks

Implement bat_fullcalendar submodule