Bump postgresql from 42.2.18 to 42.2.19 in /java/backend (#187)

Bumps [postgresql](https://github.com/pgjdbc/pgjdbc) from 42.2.18 to 42.2.19.
- [Release notes](https://github.com/pgjdbc/pgjdbc/releases)
- [Changelog](https://github.com/pgjdbc/pgjdbc/blob/REL42.2.19/CHANGELOG.md)
- [Commits](pgjdbc/pgjdbc@REL42.2.18...REL42.2.19)

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java/backend/build.gradle

@@ -32,7 +32,7 @@ dependencies {
 
     implementation 'org.jooq:jooq:3.14.8'
     // For jooq
-    jdbc 'org.postgresql:postgresql:42.2.18'
+    jdbc 'org.postgresql:postgresql:42.2.19'
 
     compile("org.springframework.boot:spring-boot-starter-jdbc")