ignite_features: Install current release of drupal/pwa

Problem/Motivation

Ignite Features depends on drupal/pwa ^2.0@RC. Version 2.0.0-rc2 is installed which is displayed as "Not supported" in the Drupal Available updates report.

Proposed resolution

The current release 2.1.0-beta6 suggests using "^2.1@beta" for the constraint.