Support composer_version: 2.5.x or warn if it is not supported

[ptmkenny]

Recently, trying to work around a composer bug, I had to revert to the 2.5 version.

Platformsh support suggested that I use composer/composer: '2.5.x' in .platform.app.yaml, which I did and which works on Platformsh.

Then I tried to get ddev to pick up this change, so I reinstalled this addon:

ddev get ddev/ddev-platformsh
ddev debug refresh

However, I I found that ddev was using composer 2.6.5, not 2.5.8.

To get ddev to use a 2.5 composer version, I had to set composer/composer: '2.5.8' in .platform.app.yaml.

It would be great to either support defining the version in the same way as platformsh (2.5.x) or to warn the user that an invalid composer version has been defined.

[rfay]

The normal way to do this right now would be composer_version: 2.5.8 in your .ddev/config.yaml.