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

Bumps [org.postgresql:postgresql](https://github.com/pgjdbc/pgjdbc) from 42.7.1 to 42.7.3.
- [Release notes](https://github.com/pgjdbc/pgjdbc/releases)
- [Changelog](https://github.com/pgjdbc/pgjdbc/blob/master/CHANGELOG.md)
- [Commits](pgjdbc/pgjdbc@REL42.7.1...REL42.7.3)

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

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

Author: dependabot[bot]

backend/build.gradle

@@ -33,7 +33,7 @@ dependencies {
     implementation 'com.fasterxml.jackson.core:jackson-databind:2.16.1'
 
     // PostgresSQL driver
-    implementation 'org.postgresql:postgresql:42.7.1'
+    implementation 'org.postgresql:postgresql:42.7.3'
 
     // Apache Commons Lang
     implementation 'org.apache.commons:commons-lang3:3.14.0'