fix: 🤔 Navigation-Item medium size name

[auroraVasconcelos]

Current behavior

The medium size name for the sd-navigation-item is 'base'.

Expected behavior

In order to keep consistence with other components and with figma the expected value would be 'md'

Steps to reproduce

Link to storybook

Technical Information

DoR

• Item has business value
• Item has been estimated by the team
• Item is clear and well-defined
• Item dependencies have been identified

DoD

• Documentation has been created/updated (if applicable)
• Migration Guide has been created/updated (if applicable)
• Relevant E2E tests (Features, A11y, Bug fixes) are created/updated
• Relevant stories (Features, A11y) are created/updated
• Implementation works successfully on feature branch