Don't use features only available since PHP 7.3

[yhabteab]

I noticed in several places where array_key_last() is used, which is available since PHP 7.3.

icinga-notifications-web/application/forms/EscalationRecipientForm.php

Line 227 in e6b64c5

$key = array_key_last($this->options);

icinga-notifications-web/application/forms/EscalationConditionForm.php

Line 260 in e6b64c5

$key = array_key_last($this->options);

icinga-notifications-web/library/Notifications/Widget/EventRuleConfig.php

Line 86 in e6b64c5

$newPosition = (int) array_key_last($escalations) + 1;

[nilmerg]

Or we consider changing the minimum requirement. To my knowledge, no distribution we support still uses PHP 7.2