chore(ci): align backport labels with namespaced label convention #2585

Context

.github/labels.yml opens with a header declaring that label names follow the Kubernetes scheme (kind/, priority/, triage/, lifecycle/, area/, do-not-merge/, security/, size/). Two labels currently violate that scheme:

• backport — top-level, no namespace. Upstream Kubernetes uses kind/backport (kubernetes/test-infra/label_sync/labels.yaml) for "issues or PRs requiring backports". Our current label means the same thing and should match.
• backport-previous — top-level, no namespace. Cozystack-specific (k8s has no two-line release model), but still needs a namespace per the file header. Reasonable options: backport/previous + a sibling backport/current (new namespace dedicated to release-line targeting), or fold into kind/ (kind/backport-previous, kind/backport-current).

Discovered while reviewing #2584 (which fixes the cascade and is unblocked by this).

Scope

Single PR, label-only. Two files to touch:

1. .github/labels.yml — rename both labels and use the aliases: mechanism so EndBug/label-sync preserves history on already-tagged PRs/issues. Also worth deciding whether the top-level release label belongs under area/release or a new namespace while the file is open.
2. .github/workflows/backport.yaml — every reference to the literal label strings 'backport' and 'backport-previous' must be updated in lockstep with the rename. References as of today: lines 21, 22, 23, 39, 40 in backport.yaml. The trigger filter in pull-requests-release.yaml is unrelated (the word "backport" there is in a comment, not a label match).

No other workflow consumes these labels.

Acceptance criteria

• .github/labels.yml namespaces both labels per the file header, with aliases: covering the old names.
• .github/workflows/backport.yaml references the new names everywhere.
• EndBug/label-sync run on the PR completes without dropping history on existing labeled PRs.
• Decision on the top-level release label is captured in the PR (either renamed or explicitly left alone with reasoning).