ci: automate release checks and publishing

[DCjanus]

Release PRs still rely on maintainers to do several manual checks and publishing
steps. We can make the process easier to review and harder to get wrong by
moving more of it into GitHub Actions.

Proposal:

• Run public API compatibility checks in CI with obi1kenobi/cargo-semver-checks-action.
• Add a release workflow that creates the GitHub release and publishes the crate after the version/changelog PR is merged.
• Publish to crates.io with Trusted Publishing via rust-lang/crates-io-auth-action, without adding a long-lived registry token.

Trusted Publishing needs one-time owner-side setup for this crate, the release
workflow file, and optionally a protected GitHub Actions environment such as
release.

[DCjanus]

@frewsxcv Sorry to bother you. Do you happen to have access to the rss crate settings on crates.io?

I can publish via the github:rust-syndication:publish team, but I don't see the Trusted Publishing setup UI. Maybe that's because I'm not a named crate owner.