Bump org.postgresql:postgresql in /modules/postgresql

Bumps [org.postgresql:postgresql](https://github.com/pgjdbc/pgjdbc) from 42.5.4 to 42.6.0.
- [Release notes](https://github.com/pgjdbc/pgjdbc/releases)
- [Changelog](https://github.com/pgjdbc/pgjdbc/blob/master/CHANGELOG.md)
- [Commits](pgjdbc/pgjdbc@REL42.5.4...REL42.6.0)

---
updated-dependencies:
- dependency-name: org.postgresql:postgresql
  dependency-type: direct:production
  update-type: version-update:semver-minor
...

Signed-off-by: dependabot[bot] <[email protected]>

Author: dependabot[bot]

modules/postgresql/build.gradle

@@ -10,7 +10,7 @@ dependencies {
     compileOnly 'io.r2dbc:r2dbc-postgresql:0.8.13.RELEASE'
 
     testImplementation project(':jdbc-test')
-    testImplementation 'org.postgresql:postgresql:42.5.4'
+    testImplementation 'org.postgresql:postgresql:42.6.0'
 
     testImplementation testFixtures(project(':r2dbc'))
     testImplementation 'io.r2dbc:r2dbc-postgresql:0.8.13.RELEASE'