[Issue] Use Variable for border-radius

[m2-assistant]

Preconditions:
This issue is automatically created based on existing pull request: #28548: Use Variable for border-radius

The border-radius was hardcoded, so i changed it to: @button__border-radius

Steps to reproduce:

Variable @button__border-radius is defined in lib/web/css/source/lib/variables/_buttons.less library and has default value for border-radius = 3px

Actual Result

Expected Result

We don't need to hardcoded value if we use the default value for the border-radius

[m2-assistant]

Hi @engcom-Alfa. Thank you for working on this issue.
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