Make "Check Image Tags (Last Release)" non-blocking

[Copilot]

Bug Fix

What was the bug?

The "Check Image Tags (Last Release)" workflow job fails when templates reference image tags that exist in the images repo's main branch but haven't been released yet. This blocks PRs like #417 (adding .NET 11 preview support) where the corresponding images PR was merged but not yet released.

How did you fix it?

Modified .github/workflows/check-image-tags.yaml to convert failures into warnings:

- name: Check image tags (last release)
  run: |
    if ! npx tsx build/check-image-tags.ts images; then
      echo "::warning::Image tag check against last release failed - templates may reference newer images not yet released"
    fi
    exit 0

This matches the existing behavior of the "Check Image Tags (Latest)" job and prevents the chicken-and-egg problem where templates can't be updated until images are released.