CI: Test e2e against all Tekton Pipelines LTS versions

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. #2388)
• A regression or behavior change in latest Pipelines (like extra WARN log output) breaks CI for unrelated PRs (e.g. #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

Start by locating the existing e2e workflow and read test/e2e-common.sh, especially its RELEASE_YAML_PIPELINE override. Compare the workflow with the tektoncd/chains example and use the Tekton Pipelines releases list to define the LTS matrix. Done means CI runs the existing e2e coverage for each LTS release alongside the current Kubernetes matrix, with the intended failure behavior.