Add master-4.28 in renovate config and cleanup renovate.json5

Author: SgtSilvio

renovate.json5

@@ -1,13 +1,14 @@
 {
-    "$schema": "https://docs.renovatebot.com/renovate-schema.json",
-    "extends": [
-        "local>hivemq/renovate-config:default.json5"
+    $schema: "https://docs.renovatebot.com/renovate-schema.json",
+    extends: [
+        "local>hivemq/renovate-config:default.json5",
     ],
     baseBranches: [
         "master",
+        "master-4.28",
         "master-4.9",
-        "renovate-playground"
+        "renovate-playground",
     ],
-    "useBaseBranchConfig": "merge",
-    "branchPrefix": "renovate/hivemq-kafka-extension-customization-sdk/"
+    useBaseBranchConfig: "merge",
+    branchPrefix: "renovate/hivemq-kafka-extension-customization-sdk/",
 }