Remove Composer1 related code

[theofidry]

No description provided.

[llaville]

Removing Composer v1 (👍 of course), but adding a so restrictive constraint to plugin API "composer-plugin-api": "^2.2".
Is it really necessary ?
I know that Composer 2.2 is now the new LTS and recommended version, but platforms that still have Composer 2.1 are not able to use box 3.15

devilbox@php-7.4.27 in /shared/backups/github/my-box $ composer update
Loading composer repositories with package information
Updating dependencies
Your requirements could not be resolved to an installable set of packages.

  Problem 1
    - Root composer.json requires composer-plugin-api ^2.2, found composer-plugin-api[2.1.0] but it does not match the constraint.

Consider it as a warning, nothing else. I'm personally be able to update my local platform to Composer 2.2

[theofidry]

I am not sure to be honest. There was a few hiccups with 2.0 to 2.2 because some (internal) files changed and this was affecting Box. So unless someone is willing to add back 2.1 and 2.0 to the build matrix and adapt the tests (as I expect some will need changes), I think it's best to stick to 2.2. At the very least I think it's a lot simpler to run a composer self update command than start this endeavor ;)