Fix Composer Setup

The latest Composer no longer matches the `sha384sum` hard-coded on line 11:

https://github.com/drupalpod/drupalpod-starterkit/blob/main/.gitpod/Dockerfile#L11

Luckily, there is [a better way](https://getcomposer.org/doc/faqs/how-to-install-composer-programmatically.md) to install the latest Composer programmatically.

Patch incoming.