Issue #3540400 by maxilein, chrisfromredfin, dydave: Fixed compatibility with...

https://www.drupal.org/project/admin_toolbar/issues/3540400

Fixed compatibility with Project Browser 2.1.x following the latest changes to the its configuration schema:

• enabled_sources was changed from an array of values to an associative array keyed by source IDs.
• Added conflict section constraint in module's composer.json file to enforce compatibility with project_browser versions greater than 2.1.0.
• Added basic Functional integration tests class to check the added extra links are displayed in the expect order.
• Updated Gitlab CI configuration to download contrib module project_browser with composer.

Additional changes:

• Normalized module's composer.json file with package ergebnis/composer-normalize.
• Added active project maintainer @DYdave.