Bump io.projectreactor:reactor-core in /modules/r2dbc

Bumps [io.projectreactor:reactor-core](https://github.com/reactor/reactor-core) from 3.8.1 to 3.8.4.
- [Release notes](https://github.com/reactor/reactor-core/releases)
- [Commits](https://github.com/reactor/reactor-core/compare/v3.8.1...v3.8.4)

---
updated-dependencies:
- dependency-name: io.projectreactor:reactor-core
  dependency-version: 3.8.4
  dependency-type: direct:production
  update-type: version-update:semver-patch
...

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