Replace GSON with Jackson ObjectMapper

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

theodolite/src/test/kotlin/rocks/theodolite/core/IOHandlerTest.kt

 package rocks.theodolite.core
 
-import com.google.gson.GsonBuilder
+import com.fasterxml.jackson.databind.ObjectMapper
 import io.quarkus.test.junit.QuarkusTest
 import org.hamcrest.CoreMatchers.containsString
 import org.hamcrest.MatcherAssert.assertThat
@@ -121,7 +121,7 @@ internal class IOHandlerTest {
             objectToSave = testContentResource
         )
 
-        val expected = GsonBuilder().enableComplexMapKeySerialization().setPrettyPrinting().create().toJson(testContentResource)
+        val expected = ObjectMapper().writerWithDefaultPrettyPrinter().writeValueAsString(testContentResource)
 
         assertEquals(
             expected,