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

Bumps [io.projectreactor:reactor-core](https://github.com/reactor/reactor-core) from 3.5.4 to 3.5.6.
- [Release notes](https://github.com/reactor/reactor-core/releases)
- [Commits](reactor/reactor-core@v3.5.4...v3.5.6)

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

Signed-off-by: dependabot[bot] <support@github.com>

Author: dependabot[bot]

modules/r2dbc/build.gradle

@@ -15,6 +15,6 @@ dependencies {
     testImplementation 'io.r2dbc:r2dbc-postgresql:0.8.13.RELEASE'
     testImplementation project(':postgresql')
 
-    testFixturesImplementation 'io.projectreactor:reactor-core:3.5.4'
+    testFixturesImplementation 'io.projectreactor:reactor-core:3.5.6'
     testFixturesImplementation 'org.assertj:assertj-core:3.24.2'
 }