roles/php-fpm: Use concrete dependencies

The php-gd, php-mysql, etc packages are meta packages that just end
up installing the concrete ones for our specific version.

roles/php-fpm/tasks/Ubuntu_20.04.yml

@@ -4,11 +4,11 @@
   - name: Set php-fpm packages
     set_fact:
       php_fpm_packages:
-        - php-fpm
+        - php7.4-fpm
         # for WordPress
-        - php-mysql
-        - php-gd
-        - php-curl
+        - php7.4-mysql
+        - php7.4-gd
+        - php7.4-curl
 
   - name: Install php-fpm and deps
     apt: name={{ php_fpm_packages }} state=present update_cache=yes