Update PHP version requirement in README

Author: BenMorel

README.md

@@ -26,9 +26,7 @@ composer require brick/postcode
 
 ## Requirements
 
-This library requires PHP 7.4 or later.
-
-For PHP 7.1, 7.2 & 7.3 compatibility, you can use version `0.2`. Note that [these PHP versions are EOL](http://php.net/supported-versions.php) and not supported anymore. If you're still using one of these PHP versions, you should consider upgrading as soon as possible.
+This library requires PHP 8.1 or later.
 
 ## Project status & release process