Refactor mobile menu to use Popover API - #4121

This PR contains the following updates:

| Package | Change | [Age](https://docs.renovatebot.com/merge-confidence/) | [Confidence](https://docs.renovatebot.com/merge-confidence/) |
|---|---|---|---|
| [@astrojs/starlight](https://starlight.astro.build) ([source](https://github.com/withastro/starlight/tree/HEAD/packages/starlight)) | [`0.41.11` → `0.42.0`](https://renovatebot.com/diffs/npm/@astrojs%2fstarlight/0.41.11/0.42.0) | ![age](https://developer.mend.io/api/mc/badges/age/npm/@astrojs%2fstarlight/0.42.0?slim=true) | ![confidence](https://developer.mend.io/api/mc/badges/confidence/npm/@astrojs%2fstarlight/0.41.11/0.42.0?slim=true) |

---

### Release Notes

<details>
<summary>withastro/starlight (@&#8203;astrojs/starlight)</summary>

### [`v0.42.0`](https://github.com/withastro/starlight/blob/HEAD/packages/starlight/CHANGELOG.md#0420)

[Compare Source](https://github.com/withastro/starlight/compare/@astrojs/starlight@0.41.11...@astrojs/starlight@0.42.0)

##### Minor Changes

- [#&#8203;3572](withastro/starlight#3572) [`292fb17`](withastro/starlight@292fb17) Thanks [@&#8203;HiDeoo](https://github.com/HiDeoo)! - Distributes package as JavaScript files with dedicated type declaration files instead of TypeScript source files.

- [#&#8203;4121](withastro/starlight#4121) [`2623ae6`](withastro/starlight@2623ae6) Thanks [@&#8203;delucis](https://github.com/delucis)! - Simplifies markup for Starlight’s mobile menu toggle

  **⚠️ Potentially breaking change:** If you use a theme plugin, custom styles, or component overrides targeting the `MobileMenuToggle` button or `PageFrame` components, you may need to adjust these for the new markup. The button is no longer wrapped in a `<starlight-menu-button>` custom element and no longer uses the `aria-expanded` attribute. Instead, you can use the `.sl-menu-button` class name to target the button and the `:popover-open` pseudo-class to style the menu open state specifically.

  In the following example, custom styles for the menu button are updated for the new approach:

  ```diff
  - starlight-menu-button button {
  + .sl-menu-button {
    color: var(--sl-color-text);
  }

  - starlight-menu-button[aria-expanded='true'] button {
  + .sl-menu-button:has(~ :popover-open) {
    color: var(--sl-color-text-accent-high);
  }
  ```

  See [`MobileMenuToggle.astro`](https://github.com/withastro/starlight/blob/main/packages/starlight/components/MobileMenuToggle.astro) and [`PageFrame.astro`](https://github.com/withastro/starlight/blob/main/packages/starlight/components/PageFrame.astro) on GitHub for the full source code of the updated components.

- [#&#8203;3572](withastro/starlight#3572) [`292fb17`](withastro/starlight@292fb17) Thanks [@&#8203;HiDeoo](https://github.com/HiDeoo)! - Removes the `tagline` configuration option, which was never used.

  If your configuration included a `tagline` option, you can safely remove it without any replacement.

- [#&#8203;4134](withastro/starlight#4134) [`6135f01`](withastro/starlight@6135f01) Thanks [@&#8203;HiDeoo](https://github.com/HiDeoo)! - Updates internal `@astrojs/mdx`, `@astrojs/markdown-satteri`, and `satteri` dependencies.

  ⚠️ **BREAKING CHANGE:** The following minimum versions are now required:

  - `astro` v7.2.10 or later
  - `@astrojs/markdown-satteri` 0.4.0 or later (if you use it)
  - `@astrojs/markdown-remark` 7.3.0 or later (if you use it)

  Please update Starlight and Astro together:

  ```sh
  npx @astrojs/upgrade
  ```

- [#&#8203;4121](withastro/starlight#4121) [`2623ae6`](withastro/starlight@2623ae6) Thanks [@&#8203;delucis](https://github.com/delucis)! - Refactors Starlight’s mobile menu toggle to work when JavaScript fails or is disabled

  ⚠️ **BREAKING CHANGE:** This release drops official support for Chromium-based browsers prior to version 116 (released August 2023), Safari-based browsers prior to version 17.0 (released September 2023), and Firefox prior to version 125 (released April 2024). You can find a list of currently supported browsers and their versions using this [browserslist query](https://browsersl.ist/#q=%3E+0.5%25%2C+not+dead%2C+Chrome+%3E%3D+116%2C+Edge+%3E%3D+116%2C+Firefox+%3E%3D+125%2C+Safari+%3E%3D+17.0%2C+iOS+%3E%3D+17.0%2C+not+op_mini+all).

  This change also removes the `data-mobile-menu-expanded` attribute, which was previously added to `<body>` while the mobile menu is open. If you have custom code that was depending on this attribute, you will need to update it to use a new selector to check if the mobile menu is open.

  In the following example, a custom background colour for the site header while the menu is open is updated for the new approach:

  ```diff
  - [data-mobile-menu-expanded] header {
  + body:has(sl-sidebar-pane:popover-open) header {
    background-color: var(--sl-color-bg);
  }
  ```

</details>

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Starlight 0.42 (PR withastro/starlight#4121) replaced the starlight-menu-button custom element and data-mobile-menu-expanded body attribute with a Popover API toggle (.sl-menu-button) and :popover-open state. Update theme selectors accordingly so the hamburger button and sidebar pane keep their dark/glass styling.