Change release notes update mode

[ecordell]

This should make it so that we can make releases directly in GH ui, and have goreleaser update it with the generated notes when it's done building.

From https://goreleaser.com/customization/release/:

  # What to do with the release notes in case there the release already exists.
  #
  # Valid options are:
  # - `keep-existing`: keep the existing notes
  # - `append`: append the current release notes to the existing notes
  # - `prepend`: prepend the current release notes to the existing notes
  # - `replace`: replace existing notes
  #
  # Default is `keep-existing`.
  mode: replace

Since we use the github-native release note generator, this should just replace the generated notes with the generate notes + our extra notes at the end.

[vroldanbet]

Nice!

It appears the change did not trigger the test-suite, and those being required means it's blocking merge