Replace GSON with Jackson ObjectMapper

As #319 (closed) suggests, replaced the use of GSON with Jackson ObjectMapper.

theodolite/src/main/kotlin/rocks/theodolite/kubernetes/slo/SloJson.kt

 package rocks.theodolite.kubernetes.slo
 
-import com.google.gson.Gson
+import com.fasterxml.jackson.databind.ObjectMapper
 import rocks.theodolite.kubernetes.util.PromResult
 
 class SloJson constructor(
@@ -9,7 +9,7 @@ class SloJson constructor(
 ) {
 
     fun toJson(): String {
-        return Gson().toJson(
+        return ObjectMapper().writeValueAsString(
             mapOf(
                 "results" to this.results,
                 "metadata" to this.metadata