Bump MINIMUM_STABILITY back to 'stable' when php-tuf/composer-stager has an official release

Problem/Motivation

We lowered MINIMUM_STABILITY to 'RC' in core/tests/Drupal/BuildTests/Composer/Template/ComposerProjectTemplatesTest.php to accommodate php-tuf/composer-stager which currently on has RC releases.

Steps to reproduce

Proposed resolution

As soon as php-tuf/composer-stager has an official release, bump MINIMUM_STABILITY back to 'stable'.

Remaining tasks

User interface changes

Introduced terminology

API changes

Data model changes

Release notes snippet