Available updates should not recommend a new branch if there are no stable releases

Problem/Motivation

I'm working on a stale 9.5.11 site that needs an core update. There's logic in update.module to not recommend releases with "extra" in their version string (alpha/beta/rc). However, right now, the 9.5.11 site is recommending 11.0.0-rc1 as the update target.

Steps to reproduce

1. Install an older, unsupported branch of core. At the time of writing, 9.5.11.
2. Make sure that the latest branch of core is marked as "Supported" on the d.o project node, but there's no official / stable release yet.
3. Visit the available updates report at /admin/reports/updates
4. Notice "Recommended version: 11.0.0-rc1 (2024-Jul-11)"

Proposed resolution

Fix the logic that's picking the "target major" for the available updates report to ignore so-called "supported" branches that don't actually have any stable releases yet.

Remaining tasks

1. Add / update tests to show the bug.
2. Fix the logic in update_calculate_project_update_status(), near this comment:
   

       // We know the current release is unsupported since it is not in
       // 'supported_branches' list. We should use the next valid supported
       // branch for the target major version.
       

User interface changes

Better recommendation when a new core branch is marked "supported" but has no stable releases.

Before

API changes

None.

Data model changes

None.

Release notes snippet

TBD.