ci: Add `data/whats_new/\w*.yml` pattern to `backend-patterns`

What does this MR do and why?

data/whats_new/\w*.yml is mapped to spec/lib/release_highlights/validator_spec.rb, which runs in the rspec unit minimal jobs. That means data/whats_new/\w*.yml needs to be part of the patterns that trigger the rspec unit minimal jobs.

Verifying this works as expected with !83380 (closed).

Also, this makes rspec:coverage gracefully exit with no coverage file.

MR acceptance checklist

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• I have evaluated the MR acceptance checklist for this MR.

Related to #356495 (closed).