feat(close): auto-close PRs targeting EOL release branches

Add a `pull_request_rules` entry that closes every PR (and Mergify-
created backport) opened against the EOL release branches `sagitta`,
`crux`, or `circinus` in any vyos/* repo extending this config.

Excludes vyos-documentation (still accepts per-branch docs backports
to sagitta and circinus) and vyos.vyos (Ansible Galaxy collection
on `main`; listed defensively).

Replaces vyos/vyos-1x:.github/workflows/pr-auto-close.yml, which only
covered sagitta. Centralizing in Mergify removes per-repo workflow
drift and extends coverage to crux and circinus.

Uses Mergify's `repository-name` condition attribute (bare repo name,
no org prefix) — unambiguous because this central config is consumed
only by vyos/* repos via `extends: mergify`.

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