Explain Unknown parsers more clearly

\Drupal\markdown\Annotation\ComposerPackage::getAvailableVersions is using a deprecated URL to check for known packages.

See https://github.com/thephpleague/commonmark/discussions/1082#discussionco... for more info.

This change doesn't explain what to do about unknown parsers, but it does:

1. fix the packagist API URL
2. only check enabled parsers
3. make the messaging a bit easier to read

As an added bonus, this fixes the Guzzle 403 error...

GuzzleHttp\Exception\ClientException: Client error: `GET https://repo.packagist.org/p/michelf/php-markdown.json` resulted in a `403 Forbidden` response: 403 Forbidden

Yes! This fixed the 403 for me. Thanks for the work on this, can we get this in a new release?