refactor: remove defaultProps usage

[matthewgallo]

Closes #5032

This PR removes any usage of defaultProps which is removed in React 19. As recommended in the React 19 upgrade guide, default props are recommended to be provided as default function parameters.

What did you change?

docs/CODE_GUIDELINES.md
packages/ibm-products/src/components/AddSelect/AddSelect.tsx
packages/ibm-products/src/components/Datagrid/Datagrid/addons/CustomizeColumns/ButtonWrapper.js
packages/ibm-products/src/components/Datagrid/Datagrid/addons/RowSize/RowSizeRadioGroup.js
packages/ibm-products/src/components/Nav/NavItemLink.js

How did you test and verify your work?

• All components still function same in storybook
• All tests pass

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