[CI] Refactor publish_draft_release.sh (#1289)

* Refactor publish_draft_release.sh

* Switch to using arrays for storing labelled changes
* Combine changes for Polkadot and Substrate
* Change sanitised_git_logs in `lib.sh` to drop `*`s at the start
* Only look for priorities of medium or above (presence of one or more
  C1 label is enforeced by check-labels anyway, saves us some API calls)

* Ensure priorities >C1-low aren't labelled B0-silent
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