Fix $border-radius-base SCSS value

What does this MR do?

This MR fixes the $border-radius-base SCSS value to 4px to align with Pajama's 8px spacing system as described in https://design.gitlab.com/layout/spacing and to match $gl-border-radius-base in gitlab-ui which is defined as 4px

Slack discussion: https://gitlab.slack.com/archives/CETG54GQ0/p1593162262093600

Screenshots

Before (dropdown uses 3px border radius):

After (dropdown uses 4px border radius):

Does this MR meet the acceptance criteria?

Conformity

• Changelog entry
• Documentation (if required)
• Code review guidelines
• Merge request performance guidelines
• Style guides
• Database guides
• Separation of EE specific content

Availability and Testing

• Review and add/update tests for this feature/bug. Consider all test levels. See the Test Planning Process.
• Tested in all supported browsers
• Informed Infrastructure department of a default or new setting change, if applicable per definition of done

Security

If this MR contains changes to processing or storing of credentials or tokens, authorization and authentication methods and other items described in the security review guidelines:

• Label as security and @ mention @gitlab-com/gl-security/appsec
• The MR includes necessary changes to maintain consistency between UI, API, email, or other methods
• Security reports checked/validated by a reviewer from the AppSec team