Bump org.testcontainers:postgresql from 1.19.3 to 1.21.3 (#1139)

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")
 }