roles/php-fpm: set user and group based on webserver

We use either caddy or nginx, which are conveniently named the same
as the Unix user and group.

roles/php-fpm/templates/php8.2-pool.conf.j2

@@ -27,8 +27,8 @@
 ;       --allow-to-run-as-root option to work.
 ; Default Values: The user is set to master process running user by default.
 ;                 If the group is not set, the user's group is used.
-user = nginx
-group = nginx
+user = {{ webserver }}
+group = {{ webserver }}
 
 ; The address on which to accept FastCGI requests.
 ; Valid syntaxes are:
@@ -52,8 +52,8 @@ listen = /run/php/php8.2-fpm-{{ domain_name }}.sock
 ; and group can be specified either by name or by their numeric IDs.
 ; Default Values: Owner is set to the master process running user. If the group
 ;                 is not set, the owner's group is used. Mode is set to 0660.
-listen.owner = nginx
-listen.group = nginx
+listen.owner = {{ webserver }}
+listen.group = {{ webserver }}
 ;listen.mode = 0660
 
 ; When POSIX Access Control Lists are supported you can set them using