Refactor Json to JsonMapper, update Gradle to 9.5.1, and set Java 26 toolchain.

src/main/java/dev/coph/nextusweb/server/json/JsonMapper.java

@@ -0,0 +1,12 @@
+package dev.coph.nextusweb.server.json;
+
+import tools.jackson.databind.ObjectMapper;
+
+
+public final class JsonMapper {
+    public static final ObjectMapper MAPPER = tools.jackson.databind.json.JsonMapper.builder()
+            // .addModule(new JavaTimeModule())
+            .build();
+    
+    private JsonMapper() {}
+}