Add support for php_version variable

[strarsis]

Trellis now uses a variable php_version for defining the PHP version: roots/trellis@5f398b2

Note: The PHP version variable can be interpolated like this: {{ php_version }}
Example of its usage in Trellis: https://github.com/roots/trellis/search?q=%7B%7B+php_version+%7D%7D

It would be therefore nice if trellis-redis also uses this variable, as in

trellis-redis/handlers/main.yml

Line 19 in 1538a4d

name: php7.4-fpm

[im-mortal]

A good idea and is actually implemented in this fork:
https://github.com/E-VANCE/trellis-redis

[strarsis]

@im-mortal, @E-VANCE: This is nice. Though having so many forks will end up in siloing these improvements.
Could this also be added to this fork? And can we somehow negotiate a singular successor fork, at least for fixes and smaller improvements?

[E-VANCE]

@strarsis Totally agree, this should be consolidated...

@im-mortal Would you be willing to carry on? If so then I'll create a PR. I'd maintain the project also, just let me know.

[im-mortal]

@strarsis, @E-VANCE, thank you for replying.

@strarsis, of course I can merge @E-VANCE's latest changes into my fork as I think using {{ php_version }} is the way.

@E-VANCE, still, I would gladly redirect to your fork if you wish to maintain it, or invite you as a collaborator if you think my fork should be up to date, or just merge your PRs as they go. I'm good either way.

[strarsis]

That would be great! Also because not all forked repositories have the issue tracker enabled.
E.g. @E-VANCE: The Trellis PHP version variable is used, but also the default PHP redis extension package is installed -
which means that sometimes also PHP 8.0 redis extension is also installed, an unnecessary extension when PHP7.x is still used.

[E-VANCE]

@im-mortal
OK, I would be happy to co-maintain so let's collaborate on your fork. I'll get the PR ready asap.

@strarsis
Good point, will look into this once we have merged the forks.

[im-mortal]

@E-VANCE Thank you! I've sent you an invite for collaboration.

[im-mortal]

In the meantime, I've opened the pull request myself:
im-mortal#2

Due to some additional changes in the fork (which don't quite belong to the scope of the pull request), I welcome any discussion there.