fix(deps): update testcontainers dependencies to specific version 1.21.4

Author: psytraxx

backend/build.gradle

@@ -34,8 +34,8 @@ dependencies {
 
 	// Test containers
 	testImplementation platform('org.testcontainers:testcontainers-bom:1.21.4')
-	testImplementation 'org.testcontainers:junit-jupiter'
-	testImplementation 'org.testcontainers:mongodb'
+	testImplementation 'org.testcontainers:junit-jupiter:1.21.4'
+	testImplementation 'org.testcontainers:mongodb:1.21.4'
 	// end::tests[]
 
 	testRuntimeOnly 'org.junit.platform:junit-platform-launcher'