PHP version update

[Shikachuu]

The PHP version should be a supported one. (So at least PHP 7.3)

Expected Behaviour

The PHP version in the templates is at least PHP version 7.3.

Current Behaviour

The current version is 7.2 which is end-of-life and no longer supported/maintained.
https://www.php.net/supported-versions.php

Are you a GitHub Sponsor (Yes/No?)

Check at: https://github.com/sponsors/openfaas

• Yes
• No

List All Possible Solutions and Workarounds

1. Update the PHP version to 7.3
2. Update the PHP version to 7.4 (Merge my PR 😈 PHP version update #266)
3. Remove the number from the PHP template, just call it php instead of php7 and migrate all the way up to PHP 8. (this is the most drastical, yet the best solution IMHO)

Which Solution Do You Recommend?

2. Seems to be the easiest (since there is a PR for it), 3. would be the best.

Steps to Reproduce (for bugs)

1. Create a functions with faas-cli with the --lang php7 flag.

Context

Create a PHP function with a secure and still maintained PHP version.

Your Environment

• What Docker version are you using? Docker version 20.10.7, build f0df350

• What version of Kubernetes are you using? none

• Operating System and version (e.g. Linux, Windows, MacOS): Linux

• Link to your project or a code example to reproduce issue: See Steps to Reproduce (for bugs)