ci: make scheduled branch matrix dynamic

.github/workflows/scheduled.yml

@@ -15,10 +15,51 @@ permissions:
   actions: write
 
 jobs:
+  # Figure out which branches to run CI on. This is the main branch, plus the
+  # three most recent release-X.Y branches (as per the support policy in
+  # RELEASES.md, we maintain latest, latest-1, and latest-2).
+  #
+  # Note this is based on branch existence rather than released versions, so
+  # from the moment a new release branch is created (at rc1, i.e. about two
+  # months before the .0 release) until that release is out, the oldest
+  # supported branch (latest-2) is not covered here. This is deliberate:
+  # testing the upcoming release is more valuable, and the branch in question
+  # is still tested on every PR and via workflow_dispatch.
+  prepare:
+    runs-on: ubuntu-latest
+    outputs:
+      branches: ${{ steps.branches.outputs.result }}
+    steps:
+      - name: Find branches to test
+        id: branches
+        uses: actions/github-script@v9
+        with:
+          result-encoding: string
+          script: |
+            const re = /^release-(\d+)\.(\d+)$/;
+            const all = await github.paginate(github.rest.repos.listBranches, {
+              owner: context.repo.owner,
+              repo: context.repo.repo,
+              per_page: 100,
+            });
+            const rel = all
+              .map((b) => b.name.match(re))
+              .filter((m) => m !== null)
+              .sort((a, b) => b[1] - a[1] || b[2] - a[2])
+              .slice(0, 3)
+              .map((m) => m[0]);
+            if (rel.length === 0) {
+              core.setFailed('No release-X.Y branches found.');
+            }
+            const branches = ['main', ...rel];
+            core.info(`Branches to test: ${branches.join(' ')}`);
+            return JSON.stringify(branches);
+
   trigger-workflow:
+    needs: prepare
     strategy:
       matrix:
-        branch: ["main", "release-1.5", "release-1.4", "release-1.3"]
+        branch: ${{ fromJSON(needs.prepare.outputs.branches) }}
         wf_id: ["validate.yml", "test.yml"]
     runs-on: ubuntu-latest
     steps: