fix: do not trigger slack notification upon adding reviewers

[mraszyk]

This PR fixes the slack notification bot to only push a notification when the PR is opened (not as a draft) or marked as ready for review. In particular, no notifications are pushed when individual reviewers are asked for a review.

[github-actions]

canbench 🏋 (dir: .) 6e84214 2025-05-22 11:13:25 UTC

✅ ./canbench_results.yml is up to date
📦 canbench_results_benchmark.csv available in artifacts

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Summary:
  instructions:
    status:   No significant changes detected 👍
    counts:   [total 304 | new 0 | improved 0 | regressed 0 | unchanged 304]
    change:   [min 0 | med 0 | max 0]
    change %: [min 0.00% | med 0.00% | max 0.00%]

  heap_increase:
    status:   No significant changes detected 👍
    counts:   [total 304 | new 0 | improved 0 | regressed 0 | unchanged 304]
    change:   [min 0 | med 0 | max 0]
    change %: [min 0.00% | med 0.00% | max 0.00%]

  stable_memory_increase:
    status:   No significant changes detected 👍
    counts:   [total 304 | new 0 | improved 0 | regressed 0 | unchanged 304]
    change:   [min 0 | med 0 | max 0]
    change %: [min 0.00% | med 0.00% | max 0.00%]

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CSV results saved to canbench_results.csv