simplify experimental flags section (#12587)

Co-authored-by: florian-lefebvre <69633530+florian-lefebvre@users.noreply.github.com>

Author: sarah11918

src/content/docs/en/guides/upgrade-to/v6.mdx

@@ -405,36 +405,34 @@ export const server = {
 
 ### Experimental Flags
 
-The following experimental flags have been removed in Astro v6.0 and these features are available for use:
-
-- `thing 1`
-
-Additionally, the following experimental flags have been removed and **are now the default or recommended behavior in Astro v6.0**.
-
-- `experimental.preserveScriptOrder` (See below for breaking changes to [default `<script>` and `<style>` behavior](#changed-script-and-style-tags-are-rendered-in-the-order-they-are-defined).)
-- `experimental.staticImportMetaEnv` (See below for breaking changes to [`import.meta.env`](#changed-importmetaenv-values-are-always-inlined).)
-
-The following experimental flags have been removed and **their corresponding features are not part of Astro v6.0**.
-
-- `thing 2`
+Experimental flags allow you to opt in to features while they are in early development. Astro may also use experimental flags to test breaking changes to default behavior. The following experimental flags have been removed in Astro 6.0 and are now stable, or the new default behavior.
 
 Remove these experimental flags from your Astro config if you were previously using them:
 
-```js del={5-7} title="astro.config.mjs"
+```js del={5-8} title="astro.config.mjs"
 import { defineConfig } from 'astro/config';
 
 export default defineConfig({
   experimental: {
+    liveContentCollections: true,
     preserveScriptOrder: true,
     staticImportMetaEnv: true,
     headingIdCompat: true,
   },
 })
 ```
 
-These features are all available by default in Astro v5.0.
+#### Experimental features now stable:
+
+- `liveContentCollections` (See the updated [content collections docs](/en/guides/content-collections/) to learn more about live collections.)
+
+#### New default or recommended behavior:
+
+- `preserveScriptOrder` (See below for breaking changes to [default `<script>` and `<style>` behavior](#changed-script-and-style-tags-are-rendered-in-the-order-they-are-defined).)
+- `staticImportMetaEnv` (See below for breaking changes to [`import.meta.env`](#changed-importmetaenv-values-are-always-inlined).)
+- `headingIdCompat` (See below for breaking changes to [Markdown heading ID generation](#changed-markdown-heading-id-generation).)
 
-<ReadMore>Read about these exciting features and more in [the v6.0 Blog post](https://astro.build/blog/astro-6/).</ReadMore>
+<ReadMore>Read about exciting new features and more in [the v6.0 Blog post](https://astro.build/blog/astro-6/).</ReadMore>
 
 
 ## Changed Defaults