ci: migrate release creation automation to release-please app

[noahdietz]

Instead of this repo using its own hand-spun release automation, it should take advantage of the Release-Please GitHub App in this Org. This will handle:

• triggering release PR creation on eligible commits
• manual release triggers
• tag/release creation
• release note generation
• semver management

We should keep any release asset generation in GitHub Actions, but have them trigger on release creation, so that they augment the release created by Release-Please.