ci: upgrade actions/checkout from v2 to v4 in knative-downstream.yaml

[Ankitsinghsisodya]

Problem

.github/workflows/knative-downstream.yaml uses actions/checkout@v2 on both the upstream and downstream checkout steps (lines 52 and 56). Version 2 is deprecated and no longer receives security patches from GitHub.

Affected File

.github/workflows/knative-downstream.yaml — lines 52 and 56

Proposed Fix

Change actions/checkout@v2 → actions/checkout@v4 on both checkout steps.

- name: Checkout Upstream
  uses: actions/checkout@v4

- name: Checkout Downstream
  uses: actions/checkout@v4

v4 is the current stable release and includes security fixes not present in v2.

[Ankitsinghsisodya]

/assign