Update composer to 2.x

[Gold]

During a deployment I noticed the following:

+++
Installing Composer...
All settings correct for using Composer
Downloading...

Composer (version 1.10.16) successfully installed to: /var/www/html/external/composer.phar
Use it: php external/composer.phar

Updating external dependencies with Composer...
Loading composer repositories with package information
Warning from https://repo.packagist.org: Support for Composer 1 is deprecated and some packages will not be available. You should upgrade to Composer 2. See https://blog.packagist.com/deprecating-composer-1-support/
+++

This looks to be coming from Makefile. Specifically `make installcomposer` which specifically downloads version 1.10.16.

[Gold (gold.catalyst)]

Mahara itself has supported Composer 2 since 2021-02-17.

See commit 6f82b14e270499c8bc9d962744c56e93747974c0

It is looking like this may be an artefact of the deployment environment.