feat: support specific composer version pinning

[gsdevme]

Seems like Composer 2.3 has dropped support for PHP 7.1, therefore now when using this action on a 7.1 project it needs to be composer 2.2 (LTS)

version - Choose which version of Composer you want to use (1 or 2)

phar_url="https://getcomposer.org"
if [ "$ACTION_VERSION" == "latest" ]
then
	phar_url="${phar_url}/composer-stable.phar"
else
	phar_url="${phar_url}/composer-${ACTION_VERSION}.phar"
fi

Problem the current locations are not the download locations, to allow for the LTS to be downloaded it seems the urls need to be changed so the follow combinations can be made.

https://getcomposer.org/download/latest-2.2.x/composer.phar
https://getcomposer.org/download/latest-2.x/composer.phar
https://getcomposer.org/download/latest-1.x/composer.phar
https://getcomposer.org/download/latest-stable/composer.phar

Given this is changing from 2 and 1 it will likely need a major version (7) release. (if acceptable)

[gsdevme]

I'm assuming if accepted a v7 would be tagged

[gsdevme]

Let me know if there is anything else Im missing here @g105b if you have time to review.

[g105b]

Hey @gsdevme , this looks great, thanks! I hadn't noticed that the support was dropped. I'll give this a test on https://github.com/php-actions/example-composer later and get this merged if all goes well.

[gsdevme]

I've given it at a test on our pipeline usage in a matrix build and its resolved the issue Im seeing 👍

The issue

Using forked version for anyone else who sees the issue for now.

[g105b]

Great, thanks so much for this. I'll make a v7 release soon.