chore(deps): bump org.postgresql:postgresql in /backend

Bumps [org.postgresql:postgresql](https://github.com/pgjdbc/pgjdbc) from 42.7.8 to 42.7.9.
- [Release notes](https://github.com/pgjdbc/pgjdbc/releases)
- [Changelog](https://github.com/pgjdbc/pgjdbc/blob/master/CHANGELOG.md)
- [Commits](pgjdbc/pgjdbc@REL42.7.8...REL42.7.9)

---
updated-dependencies:
- dependency-name: org.postgresql:postgresql
  dependency-version: 42.7.9
  dependency-type: direct:production
  update-type: version-update:semver-patch
...

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

Author: dependabot[bot]

backend/build.gradle

@@ -145,7 +145,7 @@ subprojects {
         //JDBC Drivers
         implementation 'org.xerial:sqlite-jdbc:3.51.1.0'
         implementation 'com.h2database:h2:2.4.240'
-        implementation 'org.postgresql:postgresql:42.7.8'
+        implementation 'org.postgresql:postgresql:42.7.9'
         implementation 'org.mariadb.jdbc:mariadb-java-client:3.5.6'
 
         implementation "io.ktor:ktor-client-core:$ktor_version"