[SwiftPM] Prepare guides for next stable release

[loic-sharma]

⚠️ Do not merge until after Flutter 3.27 stable release.

In the next stable release, the Swift Package Manager feature will be available on the "beta" and "stable" branches (but defaults to off). This updates the SwiftPM guides accordingly.

Depends on: https://github.com/flutter/flutter/pull/155964 Part of: https://github.com/flutter/flutter/issues/153449

Presubmit checklist

• [x] This PR is marked as draft with an explanation if not meant to land until a future stable release.
• [x] This PR doesn’t contain automatically generated corrections (Grammarly or similar).
• [x] This PR follows the Google Developer Documentation Style Guidelines — for example, it doesn’t use i.e. or e.g., and it avoids I and we (first person).
• [x] This PR uses semantic line breaks of 80 characters or fewer.