Follow on from #5058.

Changes proposed in this pull request:

• Looking at the PRs listed at the 8.1.0 draft, they mainly fall into three categories:

  • Dependencies
  • Documentation
  • Testing

• Let's configure Release Drafter to group PRs with these labels into categories with these titles

• "Deprecations" and "Removals" would be useful for the future, to make those changes clear

• Everything else falls into the blanket "Changes" section

• By design, if a PR falls into multiple categories, it'll be listed in both. If we want to avoid it, we can use explicit changelog: foo labels

• It is possible to have multiple labels per category too