[code-radiator] Skip branches with closed milestones and close stale PRs (#25761)

Two improvements to the code radiator workflow:

## Milestone-based filtering

After identifying candidate branches, the workflow now checks whether each branch has a
corresponding closed milestone. If the milestone is closed, the branch is skipped.

Milestone mapping:

| Branch pattern   | Milestone name                               |
|------------------|----------------------------------------------|
| `net<major>.0`   | `.NET <major>` (e.g., `net10.0` → `.NET 10`) |
| `xcode<version>` | `xcode<version>` (direct match)              |

## Close stale PRs superseded by manual merges

When an existing workflow-created PR is found, the workflow now checks if someone manually
merged `main` into the target branch after the PR was created. If so, the existing PR is
closed with a comment and a fresh one is created from scratch.

This handles cases like #25728 where a manual merge was done after the automated PR was
created, making the automated PR stale.

🤖 Pull request created by Copilot

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Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

Author: rolfbjarne