CI: Test e2e against all Tekton Pipelines LTS versions

[vdemeester]

Problem

The CLI e2e tests currently install only the latest Tekton Pipelines release (https://infra.tekton.dev/tekton-releases/pipeline/latest/release.yaml). This means:

• We don't catch backward compatibility issues until users report them (e.g. Failing to to tkn start with recent tkn release and old(er) pipeline version #2388)
• A regression or behavior change in latest Pipelines (like extra WARN log output) breaks CI for unrelated PRs (e.g. Add agentic workflow context files #2890)
• We have no signal on whether tkn works with older LTS releases that users are actually running

Proposal

Add a pipelines-lts e2e matrix job that tests against all Tekton Pipelines LTS versions, in addition to the existing k8s version matrix.

Prior art: tektoncd/chains

tektoncd/chains already does this well in .github/workflows/kind-e2e.yaml:

  # Latest pipelines × multiple k8s versions
  k8s:
    strategy:
      matrix:
        k8s-version: [v1.32.x, v1.33.x, v1.34.x]
    uses: ./.github/workflows/reusable-e2e.yaml
    with:
      k8s-version: ${{ matrix.k8s-version }}
      pipelines-release: v1.12.0

  # Multiple LTS pipelines × single k8s version
  pipelines-lts:
    strategy:
      matrix:
        pipelines-release:
          - v1.3.4   # LTS
          - v1.6.2   # LTS
          - v1.9.3   # LTS
    uses: ./.github/workflows/reusable-e2e.yaml
    with:
      k8s-version: v1.34.x
      pipelines-release: ${{ matrix.pipelines-release }}

The reusable workflow takes pipelines-release as input and installs from:
https://infra.tekton.dev/tekton-releases/pipeline/previous/${{ inputs.pipelines-release }}/release.yaml

Implementation notes

• The CLI already supports RELEASE_YAML_PIPELINE in test/e2e-common.sh to override the install URL — the plumbing is there
• The pipelines-lts matrix should reference pipeline releases.md for the version list
• Consider whether pipelines-lts failures should be blocking or informational initially

/cc @tektoncd/cli-maintainers