Get dependabot to check maven central

According to the docs, this shouldn't be necessary. But the Dependabot logs suggest otherwise...

json-serde/build.gradle.kts

@@ -45,6 +45,11 @@ dependencies {
 
     implementation("io.confluent:kafka-schema-registry-client:$confluentVersion")
     implementation("io.confluent:kafka-json-schema-provider:$confluentVersion")
+    constraints {
+        implementation("org.scala-lang:scala-library:2.13.12") {
+            because("lower versions have security vulnerabilities")
+        }
+    }
 
     jsonSchemaGenerator("org.creekservice:creek-json-schema-generator:$creekVersion")