v6: docs - Update utility class names in docs and examples (#42418)

v6: docs - Update utility class names in docs and examples (#42418)

Replace deprecated utility classes with the updated names across site docs and examples. Changes include switching text-* classes to fg-*, text-bg-* badge classes to theme-*, and replacing clearfix with d-flow-root. Affected files: cover and drawer-navbar examples, spinner example, reboot, accessibility, columns docs, and versions page to keep examples consistent with the new utility naming.

Add btn-icon class to navbar toggler

Add the btn-icon utility class to the navbar toggler button in the navbar-fixed example to apply icon button styling. This change updates site/src/assets/examples/navbar-fixed/index.astro and keeps the existing SVG toggler markup and collapse behavior unchanged.