Fix simple_calendar stylesheet import

- Change from @use to @import for simple_calendar CSS file
- The simple_calendar gem provides a .css file, not .scss, so @use directive fails
- Using @import allows SCSS to properly import CSS files
- Fixes SassC::SyntaxError during asset precompilation

Author: rsmithlal

app/assets/stylesheets/better_together/application.scss

@@ -36,7 +36,7 @@
 @use 'navigation';
 @use 'notifications';
 @use 'profiles';
-@use 'simple_calendar';
+@import 'simple_calendar';
 @use 'share';
 @use 'sidebar_nav';
 @use 'trix-extensions/richtext';