Create php7.4_env.yml

7.4 is being used on my system and is recommended by Nextcloud.
So I feel like this may as well be added.

defaults/php7.4_env.yml

+---
+# [system]
+# system_configuration for php 7.4:
+php_dir: "/etc/php/7.4"
+php_bin: "php-fpm7.4"
+php_pkg_apcu: "php-apcu"
+php_pkg_spe:
+  - "php-imap"
+  - "php-imagick"
+  - "php-redis"
+  - "php7.4-xml"
+  - "php7.4-zip"
+  - "php7.4-mbstring"
+php_socket: "/run/php/php7.4-fpm.sock"