Update to composer v2

[chillu]

There seems to be some coupling between running the global composer binary/phar, and including the PHP libraries, as evidenced by this build failure: https://travis-ci.com/github/silverstripe/addons.silverstripe.org/builds/202246360#L463. I'm fixing this now via falling back to v1 in travis.yml

Functionally, it's unclear how long composer v1 will be supported.

[robbieaverill]

We probably need to update those module rating repos to work with Composer 2!

[chillu]

PR at #255, side effect from investigation into OOM errors on prod.

We probably need to update those module rating repos to work with Composer 2!

Done that now: silverstripe/moduleratings-plugin#2. But I've also removed the dependency from addons, we don't seem to need the composer plugin, just silverstripe/moduleratings which doesn't have any direct composer dependencies (no upgrade necessary)

[robbieaverill]

Yep, you're right. The plugin is there so you can run the module rating commands in the addons project from the command line, if you want to test it. I also used the plugin to moduleratings library when I was building it. You can remove it and use BuildTasks instead though.

[chillu]

Merged and pushed to prod