fix: update Font Awesome icon classes to FA6 format

[praaatap]

Update icon class prefix from deprecated FA4 'fa' to FA6 'fas' in alert blocks and custom Liquid blocks. Additionally, corrects a typo 'fa-fa-hands-wash' (double prefix) that was preventing the Disclaimer icon from displaying properly

Description

This Pull Request will update the Font Awesome icon prefixes throughout the site's alert data and custom Liquid blocks to reflect the new FA6 style. Additionally, this PR will correct a specific typo that was causing the Disclaimer icon to not display properly on the site.

Changes Made

• Fixed Icon Prefix: Updated the deprecated Font Awesome 4 fa prefix to the new Font Awesome 6 fas (solid) prefix in [_data/alerts.yml]and [_plugins/customblocks.rb]
• Fixed Redundant Prefix Typo: Fixed the fa-fa-hands-wash typo in [_data/alerts.yml](line 5). The double prefix was preventing the icon from rendering properly on the live site.
• Updated Deprecated Aliases: Updated fa-warning to fa-exclamation-triangle to reflect the new Font Awesome naming conventions. ## Why this is important
  Using the new fas prefixes in the latest Font Awesome 6 library will provide better performance and consistency across modern browsers. More importantly, this PR will fix the broken Disclaimer icon due to a high-impact typo.

[MakisH]

Thanks for the contribution, but this PR is not clearly motivated.

Both the fa and fas are valid. The fas is with filling, the fa without. It is not related to the version, and the alerts currently work.

There used to be a typo in the disclaimer alert, but this is not the case anymore.