Use more consistent composer version for dev/CI tools

Problem/Motivation

Follow-up to #349

The version of composer used in our various dev and CI tools is currently pretty inconsistent and in some cases, non-deterministic.

Lando, DDEV and Probo all provide an older 1.x install of composer in their Docker images.
We currently run composer self-update in our ProboCI configs which means that the version of composer used in our ProboCI builds varies over time.
For our GitHub actions we're currently relying on the version of composer installed on their virtual environment (currently 2.0.6): https://github.com/actions/virtual-environments/blob/main/images/linux/Ubuntu1804-README.md#php

It would be nice if we used the same version (or at least approximately the same version) everywhere.

For lando we should consider using a recently-added feature that allows a composer version to be specified in a .lando.yml config file (see https://github.com/lando/lando/issues/2658)
For Probo and DDEV (and possibly GitHub actions) we might want to consider using composer's self-update command with a specific version argument, e.g.:
```
composer self-update 2.0.7
```