_ci_: Switches goreleaser notes back to default (keep-existing)

[ianconsolata]

I tried out goreleaser manually on the 1.17.0 release, and it appended a huge changelog that included duplicates of what was already there. This changes goreleaser so it will leave the existing release notes as-is if it finds them.

Before you mark the PR ready for review, please make sure that:

• All commits have a clear commit message.
• The PR title is in the form of of <PR type>: <area>: <change being made>
  • example: fix: mempool: Introduce a cache for valid signatures
  • PR type: fix, feat, INTERFACE BREAKING CHANGE, CONSENSUS BREAKING, build, chore, ci, docs,perf, refactor, revert, style, test
  • area: api, chain, state, vm, data transfer, market, mempool, message, block production, multisig, networking, paychan, proving, sealing, wallet, deps
• This PR has tests for new functionality or change in behaviour
• If new user-facing features are introduced, clear usage guidelines and / or documentation updates should be included in https://lotus.filecoin.io or Discussion Tutorials.
• CI is green