ci(release): Use action-prepare-release and rework dry-run (#761)

Switches to using getsentry/action-prepare-release for common steps and removes the dry run option from the prepare stage as it prevents us from testing the publish step. Prepare step should be safe anyways and can be skipped with the skip-prepare input.

Related: https://app.asana.com/0/1198192131329257/1198192131329301, https://app.asana.com/0/1198192131329257/1198192573081717