Development instructions require unavailable composer version

The [development instructions](https://github.com/Automattic/jetpack/blob/master/docs/development-environment.md#composer) specify installing composer 2.0 via Homebrew, and `tools/check-development-environment.sh` complains about composer 2.1.x being installed, but composer 2.0 is **no longer available** via Homebrew.

I suggest either broadening our compatibility or offering instructions on downloading the older version.

Read docs/development-environment.md at the Composer section and tools/check-development-environment.sh, then verify the available Composer versions and the checker's current requirement. Done means the documented setup and environment check agree on a Composer version that contributors can obtain, or clearly explain how to install the required older version.

Written by the indexing model from the issue text.