Install or upgrade CiviCRM using Composer 2?
It should be possible to do any one of these:

Use CiviCRM 5.31+. The error message above suggests it is using 5.30.x, which has older metadata. (Compare: 5.31's composer.json vs 5.30's composer.json)

Stick to composer v1 for as long as you're using an older version of Civi.
composer self-update --1

In the future, after you switch to a newer version of Civi (eg 5.31+), you can continue to composer v2:
composer self-update --2


Coerce the versions. Tell composer to install a new version of composer-downloads-plugin (v3.0.0 -- which supports composer v2) and pretend like it's an old version (v2.1.99 -- which is expected in civicrm-core v5.30). Command:
composer require 'civicrm/composer-downloads-plugin:v3.0.0 as v2.1.99'

In the future, after you switch to a newer version of Civi (eg 5.31+), you should edit composer.json and remove the coerced constraint. Otherwise, you may see weird behavior.