Update changelog-helper.sh to emit "Skipping PR #pr (...title...)" when SKIP encountered

[pljones]

Another change request for the changelog-helper:

$ grep SKIP ./tools/changelog-helper.sh
        if [[ "${title_suggestion_in_pr}" == "SKIP" ]]; then

Could do with this issuing a message. I got

Checking if all PR references in git log since r3_11_0 are included for 3.12.0 based on ref origin/main...
-> Ignoring PR #3512, which was mentioned in 'git log r3_11_0..HEAD', but already has milestone 'Release 3.11.0'

on the first pass. I checked #3512 and it was mis-tagged, so I fixed it. When I reran, I didn't see it mentioned. That caused me to restore the updated ChangeLog and start again. It still didn't appear -- so then I checked the PR and it was tagged "SKIP". Having

-> Skipping PR #3512 (...title...)

would have been more reassuring.

Originally posted by @pljones in #3443