Prepare this repository for Bootstrap v6

[julien-deramond]

Summary

Sets up the v6-dev branch per #996, so this repo tracks Bootstrap's v6-dev branch the same way it tracks the stable release on main.

• All 9 examples now depend on bootstrap via github:twbs/bootstrap#v6-dev (v6 alpha isn't on npm/CDN yet) and swap @popperjs/core for @floating-ui/dom.
• Sass moved from @import to @use, with variable overrides ported to v6's config/theme token system.
• Removed the Sass deprecation-silencing workarounds each example carried under v5 (silenceDeprecations in webpack.config.js/vite.config.js, and parcel/.sassrc.js) — those existed only to hide warnings from v5's legacy @import-based Sass, which v6's @use-based Sass doesn't trigger. Verified npm run build is warning-free in every example after removing them.
• Markup and classes updated per Bootstrap's official v5-to-v6 migration guide: offcanvas → drawer, dropdown → menu (including visible copy, not just classes), button/text/background utility renames, fs-1..6 → t-shirt sizes, and the navbar's collapse toggle now opens a drawer.
• Fixed a dark-mode bug the migration surfaced: the old --body-color/--body-bg override doesn't exist as a real v6 token (it's --fg-body/--bg-body, generated from $theme-fgs/$theme-bgs), so it was a silent no-op — on a system with a dark OS preference, examples rendered with dark-mode text over a light background. Fixed by using the real token names and pinning data-bs-theme="light" where the example intends one fixed light look; color-modes instead drops the override entirely since it needs --bg-body/--fg-body to keep following its own picker.
• Docs links point at the v6-dev Netlify preview (v6-dev--twbs-bootstrap.netlify.app) instead of getbootstrap.com/docs/5.3.
• READMEs (including root) call out that this branch previews an alpha and link back to the Bootstrap 5 version on main.

react-nextjs is removed from this branch (still present on main). It depends on React Bootstrap, which has no Bootstrap v6-compatible release yet — its 3.0.0-beta line still targets Bootstrap 5 markup — and since its components render their own DOM, pairing it with v6's CSS breaks styling entirely (verified: Offcanvas/Dropdown rendered completely unstyled). Rather than ship a broken example or leave a Bootstrap-5-only outlier sitting in an otherwise-v6 branch, it's removed here until either React Bootstrap ships v6 support, or it's rewritten to use plain Bootstrap markup instead (like every other example in this repo already does), dropping the React Bootstrap dependency.

Test plan

• npm install && npm test/npm run build passes with zero warnings (Sass or otherwise) in each example
• Manually exercised drawer/menu/popover interactions and color-mode switching in a browser for each
• Verified the dark-mode fix by forcing the browser into a dark OS color scheme and re-checking every example
• Maintainer review of the v6 class/markup choices (several, like the theme-color override shape and bg-2 in place of bg-light, are judgment calls made without an official migration example to copy)
• Decide react-nextjs's path forward (wait on React Bootstrap vs. rewrite without it)

🤖 Generated with Claude Code

[julien-deramond]

This PR is not to be merged as it will live as v6-dev until we release Bootstrap v6 alpha but @mdo, could you please check the content, rendering, and DOM modifications in this PR so that we agree this v6-dev branch can be at some point the main branch. 🙏