[TASK] Deprecate support for PHP 7.2 and 7.3 (MyIntervals#1565)

WordPress 7.0 will drop support for PHP 7.2 and 7.3, [1] and
PHP 7.3 was EOLed in December 2021. [2]

So it's time to deprecate support for PHP < 7.4, allowing us
to make use of features like native types for properties, as
discussed. [3]

[1] https://make.wordpress.org/core/2026/01/09/dropping-support-for-php-7-2-and-7-3/
[2] https://www.php.net/eol.php
[3] MyIntervals#1563

Author: oliverklee

CHANGELOG.md

@@ -14,6 +14,8 @@ Please also have a look at our
 
 ### Deprecated
 
+- Deprecate support for PHP 7.2 and 7.3 (#1565)  
+
 ### Removed
 
 ### Fixed