Bump nostr-sdk from 0.27.0 to 0.28.0

Bumps [nostr-sdk](https://github.com/rust-nostr/nostr) from 0.27.0 to 0.28.0.
- [Release notes](https://github.com/rust-nostr/nostr/releases)
- [Commits](rust-nostr/nostr@v0.27.0...v0.28.0)

---
updated-dependencies:
- dependency-name: nostr-sdk
  dependency-type: direct:production
  update-type: version-update:semver-minor
...

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