Place the PHP version into a variable

[binarykitchen]

Sooner or later, when upgrading Nextcloud, we will have to upgrade PHP as well.

As a preparation, to make any future PHP upgrades easier, I recommend putting its version into a variable first.

For example:

server unix:/var/run/php/php8.0-fpm.sock;

Should actually come from a variable, like:

server unix:/var/run/php/php{{phpver}}-fpm.sock;

For inspiration, refer to these commits from other forks:

• 35afe79
• 1ae8436

Again, in this ticket, make no PHP upgrades. Just have the PHP version defined in one place only.

[binarykitchen]

PR is here: #2308