Bump org.testcontainers:postgresql from 1.19.3 to 1.21.3

Bumps [org.testcontainers:postgresql](https://github.com/testcontainers/testcontainers-java) from 1.19.3 to 1.21.3.
- [Release notes](https://github.com/testcontainers/testcontainers-java/releases)
- [Changelog](https://github.com/testcontainers/testcontainers-java/blob/main/CHANGELOG.md)
- [Commits](testcontainers/testcontainers-java@1.19.3...1.21.3)

---
updated-dependencies:
- dependency-name: org.testcontainers:postgresql
  dependency-version: 1.21.3
  dependency-type: direct:production
  update-type: version-update:semver-minor
...

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

Author: dependabot[bot]

appsignals-tests/contract-tests/build.gradle.kts

@@ -56,8 +56,8 @@ dependencies {
   testImplementation("software.amazon.awssdk:sts")
   testImplementation(kotlin("test"))
   implementation(project(":appsignals-tests:images:grpc:grpc-base"))
-  testImplementation("org.testcontainers:kafka:1.19.3")
-  testImplementation("org.testcontainers:postgresql:1.19.3")
+  testImplementation("org.testcontainers:kafka:1.21.3")
+  testImplementation("org.testcontainers:postgresql:1.21.3")
   testImplementation("org.testcontainers:mysql:1.19.8")
   testImplementation("com.mysql:mysql-connector-j:8.4.0")
 }