up php default version to 7.4

.env.example

@@ -39,7 +39,7 @@ COMPOSE_PROJECT_NAME=laradock
 
 # Select a PHP version of the Workspace and PHP-FPM containers (Does not apply to HHVM).
 # Accepted values: 8.0 - 7.4 - 7.3 - 7.2 - 7.1 - 7.0 - 5.6
-PHP_VERSION=7.3
+PHP_VERSION=7.4
 
 ### Phalcon Version ###########################################