T8947: migrate remaining current→rolling residuals (rolling branch) (#1206)

* T8947: migrate remaining current→rolling residuals on rolling branch

Post-1c the default branch is `rolling`, but several build-config values
on this branch still referenced the old `current` repo path / codename /
train identity. These are not cosmetic — they break the rolling ISO build:

- docker/vyos-dev.list: apt source baked into vyos/vyos-build:rolling still
  pointed at repositories/current (Hit/Err on package builds). Now
  repositories/rolling + codename `rolling` (verified against live repo:
  Suite/Codename `rolling`, Components `main`).
- data/defaults.toml: vyos_mirror → repositories/rolling; vyos_branch →
  rolling (build-vyos-image checks out vyos-1x at this branch — `current`
  no longer exists post-1c); release_train → rolling (apt pin codename +
  os-release identity).
- data/live-build-config/archives/vyos-dev.pref.chroot: Pin codename
  n=rolling to match the repo.
- data/versions: key current→rolling. build-vyos-image looks up
  version_mapping[git_branch]; git_branch is `rolling`, so the `current`
  key KeyErrored and the version fell back to 999.<timestamp>.
- .github/config/smoketest-branches.json: rolling-train entry key
  current→rolling (looked up by branch name; was already 404ing on
  rolling) + mirror + container_image to rolling.

os-release impact (intended): rolling images now report
VERSION_CODENAME=rolling and "(rolling)" in PRETTY_NAME/VERSION. The
-rolling version suffix is hardcoded and unaffected.

Companion to Rollout 1c (T8943). Ref IS-504.

🤖 Generated by [robots](https://vyos.io)

* T8947: finish current→rolling residuals in docs/Dockerfile examples

Address adversarial-review findings (Codex) — stale current refs on the
rolling branch outside build config:

- .github/PULL_REQUEST_TEMPLATE.md: CONTRIBUTING link vyos-1x/blob/current
  → blob/rolling (was a broken link — vyos-1x renamed current→rolling in 1c).
- README.md: "Development Branches" section described the default branch as
  `current`; updated to `rolling` (dropped the obsolete "may switch to main"
  note — the rename already happened, to rolling).
- docker/Dockerfile: build-example comments vyos-build:current* →
  vyos-build:rolling*.

Ref IS-504, companion to T8943.

🤖 Generated by [robots](https://vyos.io)

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