Bump org.postgresql:postgresql from 42.5.0 to 42.5.5 in /backend

Bumps [org.postgresql:postgresql](https://github.com/pgjdbc/pgjdbc) from 42.5.0 to 42.5.5.
- [Release notes](https://github.com/pgjdbc/pgjdbc/releases)
- [Changelog](https://github.com/pgjdbc/pgjdbc/blob/master/CHANGELOG.md)
- [Commits](pgjdbc/pgjdbc@REL42.5.0...REL42.5.5)

---
updated-dependencies:
- dependency-name: org.postgresql:postgresql
  dependency-type: direct:production
...

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

Author: dependabot[bot]

backend/pom.xml

@@ -21,7 +21,7 @@
         <javalin.version>6.4.0</javalin.version>
         <jackson.version>2.18.2</jackson.version>
         <slf4j-api.version>2.0.16</slf4j-api.version>
-        <postgresql.version>42.5.0</postgresql.version>
+        <postgresql.version>42.5.5</postgresql.version>
         <hikaricp.version>5.0.1</hikaricp.version>
         <exposed.version>0.44.0</exposed.version>
         <logback.version>1.5.0</logback.version>