This issue is automatically created based on existing pull request: #30923: Clean unneccesary prefix flexbox

Description (*)

Magento now support IE 11+ (mgt2.3) and major browsers update support flexbox so i think it's time for clear prefix flexbox
Only clear prefix and keep name mixins for maintain compatible

2.3.x browser support
2.4.x browser support

Related Pull Requests

Fixed Issues (if relevant)

1. Fixes magento/magento2#<issue_number>

Manual testing scenarios (*)

1. ...
2. ...

Questions or comments

Contribution checklist (*)

• Pull request has a meaningful description of its purpose
• All commits are accompanied by meaningful commit messages
• All new or changed code is covered with unit/integration tests (if applicable)
• All automated tests passed successfully (all builds are green)