Unable to update PHP dependencies via PHP Composer in Plesk: Support for composer 1 is deprecated

Symptoms

Unable to update PHP dependencies via PHP Composer in Plesk:

PLESK_ERROR: Support for composer 1 is deprecated

Cause

This is bug EXTPHPCOMP-188 that will be fixed in future updates. 

Resolution

1. Log in to Plesk
2. Go to Extensions > search panel.ini editor > click Get it free
3. Go to Extensions > My Extensions > Panel.ini editor > Edit and add the following strings to the end of the file:
   

   [ext-composer]
   commandInfoPluginVersion = "~1.1.0"
   ComposerVersion = '2'

4. Go to Tools & Settings > Scheduled tasks (Cronjobs) and click Add task. Paste the following command and click Run now:
   

   /opt/plesk/php/7.4/bin/php /opt/psa/var/modules/composer/composer.phar self-update --2

   Note: Use actual PHP version in the command above to update PHP composer