Document the raised browser floors for upgraders

The migration guide never mentioned browser support. That was the largest
gap in it, because v5 supported Chrome and Firefox 60 and Safari 12, while
v6 now needs Chrome and Edge 130, Firefox 132, and Safari 18. A reader with
an older audience cannot adopt v6 at all, and nothing in the guide said so.

Add a warning callout to the upgrade steps rather than a changelog bullet,
because this is a prerequisite to check before starting, not a change to
apply while working. It links to the browsers and devices page for the
reason behind each version.

The approach page embeds `.browserslistrc` in full, so the rationale
comments added in the last commit now render there ahead of the queries.
Point that page at the summary table, so a reader who only wants the
versions does not have to scroll the comments.

Author: mdo

site/src/content/docs/getting-started/approach.mdx

@@ -168,7 +168,7 @@ For improved cross-browser rendering, we use [Reboot]([[docsref:/content/reboot]
 
 ### Browser support
 
-You can find our supported range of browsers and their versions [in our `.browserslistrc` file]([[config:repo]]/blob/v[[config:current_version]]/.browserslistrc):
+You can find our supported range of browsers and their versions [in our `.browserslistrc` file]([[config:repo]]/blob/v[[config:current_version]]/.browserslistrc). The comments record why each floor sits where it does, and which platform features the floors do and do not give us. For a summary table instead, see [Browsers & devices]([[docsref:/getting-started/browsers-devices]]).
 
 <Code lang="plaintext" filePath=".browserslistrc" />
 

site/src/content/docs/guides/migration.mdx

@@ -50,6 +50,10 @@ Bootstrap 6 is a major release with many breaking changes to modernize our codeb
 
 5. Recompile your Sass to see the changes.
 
+<Callout type="warning">
+**Check your browser support first.** Bootstrap 6 raises the minimum browser versions sharply: Chrome and Edge 130, Firefox 132, and Safari 18 on both macOS and iOS. Bootstrap 5 supported Chrome and Firefox 60, and Safari 12. We build on `light-dark()`, `:has()`, container queries, `content-visibility`, and the native `<dialog>` element, and v6 ships no fallbacks, prefixes, or polyfills below the floor. Pages will not render correctly in older browsers. Check your own analytics before you upgrade, and see [Browsers & devices]([[docsref:/getting-started/browsers-devices]]) for the full policy and the reason behind each version.
+</Callout>
+
 ## Changelog
 
 ### CSS