ci: dynamically choose correct release branch

Choose correct release branch (e.g. `v1` or `v2`) based on the released version. Determine the major version from `GITHUB_REF` and push to the corresponding release branch.

Switch to strategy `fail` as merges into the release branch should neither necessary nor desirable.

.github/workflows/publish.yml

@@ -53,21 +53,28 @@ jobs:
           git_committer_name: ${{ secrets.BUMP_GIT_NAME }}
           git_committer_email: ${{ secrets.BUMP_GIT_EMAIL }}
           github_token: ${{ secrets.GH_TOKEN }}
-  sync_main_to_v1:
+  sync_release_to_vx:
     if: |
       always() &&
       (needs.semantic-release.result == 'success' || needs.semantic-release.result == 'skipped') &&
-      (startsWith(github.ref, 'refs/tags/v1') && !contains(github.ref, 'beta'))
-    name: Sync branch `main` to `v1` (fast-forward enabled)
+      (startsWith(github.ref, 'refs/tags/v') && !contains(github.ref, 'beta'))
+    name: Sync new release to `vx`-branch
     runs-on: ubuntu-latest
     needs:
       - semantic-release
     steps:
-      - name: Keep `v1` up to date with `main` (fast-forward enabled)
+      - name: Get version
+        id: version
+        run: echo ::set-output name=version::${GITHUB_REF#refs/tags/}
+      - name: Get major version
+        id: major
+        run: version=${{ steps.version.outputs.version }} && echo ::set-output name=major::${version%%.*}
+      - name: Keep `vx` up to date with new release
         uses: jojomatik/sync-branch@beta
         with:
-          source: "main"
-          target: "v1"
+          source: ${{ github.ref }}
+          target: ${{ format('refs/heads/{0}', steps.major.outputs.major) }}
+          strategy: "fail"
           git_committer_name: ${{ secrets.BUMP_GIT_NAME }}
           git_committer_email: ${{ secrets.BUMP_GIT_EMAIL }}
-          github_token: ${{ secrets.GH_TOKEN }}
+          github_token: ${{ secrets.GH_TOKEN }}