ci: apply private-port-next label to any v<N>-next merge-train base (v5-next) (#24538)

Follow-up to the merged v6 merge-train wiring (#24516 / #24517).
Generalizes the `private-port-next` auto-label in
`merge-train-create-pr.yml` from `v5-next`-only to any `v<N>-next` base,
so `merge-train/spartan-v6` PRs into `v6-next` are automatically
forward-ported by the private-port-next driver — matching the agreed v6
plan (v6-next work is forward-ported, not held).

```
-              if [[ "$base_branch" == "v5-next" ]]; then
+              if [[ "$base_branch" =~ ^v[0-9]+-next$ ]]; then
```

No change to v5 behavior (`v5-next` still matches). Hand-ported onto
`v5-next` so a `v6-next` forked off it carries the rule immediately;
companion PR opened against `next` to keep the two workflow copies
identical.

---
*Created by
[claudebox](https://claudebox.work/v2/sessions/706da64a13de6c13) ·
group: `slackbot`*

Author: AztecBot

.github/workflows/merge-train-create-pr.yml

@@ -55,8 +55,9 @@ jobs:
               if [[ "$branch" == "merge-train/spartan" || "$branch" == "merge-train/spartan-v5" || "$branch" == "merge-train/spartan-v6" || "$branch" == "merge-train/ci" ]]; then
                 labels="$labels,ci-full-no-test-cache"
               fi
-              # Trains targeting the v5 release line are ports into v5-next.
-              if [[ "$base_branch" == "v5-next" ]]; then
+              # Trains targeting a v<N>-next release line (v5-next, v6-next) are
+              # forward-ported into their base via the private-port-next driver.
+              if [[ "$base_branch" =~ ^v[0-9]+-next$ ]]; then
                 labels="$labels,private-port-next"
               fi
               gh pr create --base "$base_branch" --head "$branch" \