ci: update Scylla test version from 2025.2 to 2026.1

The integration test suite was pinned to release:2025.2 which is no
longer the latest LTS branch.  Update to release:2026.1 so CI covers
the newest ScyllaDB features and catches regressions earlier.

Tests gated by @skip_scylla_version_lt(2026.1.0) — such as the
client_routes tests — will now actually execute in CI.

Author: mykaul

.github/workflows/integration-tests.yml

@@ -38,7 +38,7 @@ jobs:
     if: "!contains(github.event.pull_request.labels.*.name, 'disable-integration-tests')"
     runs-on: ubuntu-24.04
     env:
-      SCYLLA_VERSION: release:2025.2
+      SCYLLA_VERSION: release:2026.1
     strategy:
       fail-fast: false
       matrix: